diff --git a/.travis.yml b/.travis.yml
index b276d4fbd..b67478eae 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: ruby
-sudo: false
+os: linux
 
-ruby_supported_versions:
+_ruby_supported_versions:
   - &ruby_2_1 2.1.10
   - &ruby_2_2 2.2.10
   - &ruby_2_3 2.3.8
@@ -10,15 +10,15 @@ ruby_supported_versions:
   - &ruby_2_6 2.6.5
   - &ruby_head ruby-head
 
-jruby_supported_versions:
+_jruby_supported_versions:
   - &jruby_9_1 jruby-9.1.17.0
   - &jruby_9_2 jruby-9.2.8.0
   - &jruby_head jruby-head
 
-jdk_supported_versions:
+_jdk_supported_versions:
   - &jdk_8 openjdk8
 
-rails_supported_versions:
+_rails_supported_versions:
   - &rails_4_1 RAILS_VERSION=4.1
   - &rails_4_1_jruby RAILS_VERSION=4.1 JRUBY_OPTS='--dev -J-Xmx1024M --debug'
   - &rails_4_2 RAILS_VERSION=4.2
@@ -41,7 +41,7 @@ cache:
 before_install:
   - "travis_retry gem update --system 2.7.9"
   - "travis_retry gem install bundler -v '1.17.3'"
-install: BUNDLER_VERSION=1.17.3 bundle install --path=vendor/bundle --retry=3 --jobs=3
+install: bundle install --path=vendor/bundle --retry=3 --jobs=3
 
 script:
   - bundle exec rake ci
@@ -49,7 +49,9 @@ after_success:
   - codeclimate-test-reporter
 
 env:
-  matrix:
+  global:
+    - BUNDLER_VERSION=1.17.3
+  jobs:
     - *rails_4_1
     - *rails_4_2
     - *rails_5_0
@@ -70,7 +72,7 @@ rvm:
 branches:
   only: 0-10-stable
 
-matrix:
+jobs:
   include:
     - { rvm: *jruby_9_1,  jdk: *jdk_8, env: *rails_4_1_jruby }
     - { rvm: *jruby_9_1,  jdk: *jdk_8, env: *rails_4_2_jruby }
diff --git a/active_model_serializers.gemspec b/active_model_serializers.gemspec
index 8630cd324..a57b8d9a8 100644
--- a/active_model_serializers.gemspec
+++ b/active_model_serializers.gemspec
@@ -59,5 +59,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'timecop', '~> 0.7'
   spec.add_development_dependency 'grape', ['>= 0.13', '< 0.19.1']
   spec.add_development_dependency 'json_schema'
-  spec.add_development_dependency 'rake', ['>= 10.0', '< 12.0']
+  spec.add_development_dependency 'rake', ['>= 10.0', '< 13.0']
 end
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 605d9dc5e..e3f973a34 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -73,7 +73,7 @@ def self.serializer_lookup_chain_for(klass, namespace = nil)
     # Used to cache serializer name => serializer class
     # when looked up by Serializer.get_serializer_for.
     def self.serializers_cache
-      @serializers_cache ||= ThreadSafe::Cache.new
+      @serializers_cache ||= Concurrent::Map.new
     end
 
     # @api private
diff --git a/lib/active_model/serializer/reflection.rb b/lib/active_model/serializer/reflection.rb
index 9c41bad83..170ea7713 100644
--- a/lib/active_model/serializer/reflection.rb
+++ b/lib/active_model/serializer/reflection.rb
@@ -87,8 +87,8 @@ def initialize(*)
       #       meta ids: ids
       #     end
       #   end
-      def link(name, value = nil)
-        options[:links][name] = block_given? ? Proc.new : value
+      def link(name, value = nil, &block)
+        options[:links][name] = block_given? ? block : value
         :nil
       end
 
@@ -102,8 +102,8 @@ def link(name, value = nil)
       #       href object.blog.id.to_s
       #       meta(id: object.blog.id)
       #     end
-      def meta(value = nil)
-        options[:meta] = block_given? ? Proc.new : value
+      def meta(value = nil, &block)
+        options[:meta] = block_given? ? block : value
         :nil
       end
 
diff --git a/test/serializers/serializer_for_with_namespace_test.rb b/test/serializers/serializer_for_with_namespace_test.rb
index 3bba5e4af..f0f84528d 100644
--- a/test/serializers/serializer_for_with_namespace_test.rb
+++ b/test/serializers/serializer_for_with_namespace_test.rb
@@ -35,12 +35,11 @@ class PublisherSerializer < ActiveModel::Serializer
       class BookSerializer < ActiveModel::Serializer
         attributes :title, :author_name
       end
+
       test 'resource without a namespace' do
         book = Book.new(title: 'A Post', author_name: 'hello')
 
-        # TODO: this should be able to pull up this serializer without explicitly specifying the serializer
-        #       currently, with no options, it still uses the Api::V3 serializer
-        result = ActiveModelSerializers::SerializableResource.new(book, serializer: BookSerializer).serializable_hash
+        result = ActiveModelSerializers::SerializableResource.new(book).serializable_hash
 
         expected = { title: 'A Post', author_name: 'hello' }
         assert_equal expected, result