Fix all Rubocop warnings.

Rakefile

@@ -7,4 +7,4 @@ Rake::TestTask.new do |t|
   t.verbose = false
 end
 
-task :default => :test
+task default: :test

test/test_helper.rb

@@ -1,12 +1,12 @@
 require "minitest/autorun"
 require "yaml"
 
-IMAGE_EXTENSIONS = [".jpg", ".jpeg", ".png"].freeze
+IMAGE_EXTENSIONS = %w[.jpg .jpeg .png].freeze
 
-VALID_METADATA_KEYS = ["aliases", "created_by", "display_name", "github_url", "logo", "related",
-                       "released", "short_description", "topic", "url", "wikipedia_url"].freeze
+VALID_METADATA_KEYS = %w[aliases created_by display_name github_url logo related
+                         released short_description topic url wikipedia_url].freeze
 
-REQUIRED_METADATA_KEYS = ["topic", "short_description"].freeze
+REQUIRED_METADATA_KEYS = %w[topic short_description].freeze
 
 def topics_dir
   File.expand_path("../topics", File.dirname(__FILE__))
@@ -39,8 +39,8 @@ def metadata_for(topic)
   parts = File.read(path).split("---", 3)
   return unless parts.size >= 2
 
-  metadata = begin
-    YAML.load(parts[1])
+  begin
+    YAML.safe_load(parts[1])
   rescue Psych::SyntaxError => ex
     flunk "invalid YAML: #{ex.message}"
   end

test/topics_test.rb

@@ -16,7 +16,7 @@
 
         if path = paths.first
           assert_equal topic, File.basename(path, File.extname(path)),
-            "expected image to be named [topic].[extension]"
+                       "expected image to be named [topic].[extension]"
         end
       end
 
@@ -38,7 +38,7 @@
         if File.file?(path)
           lines = File.readlines(path)
 
-          assert lines.size > 0
+          assert !lines.empty?
           assert_equal "---\n", lines[0], "expected file to start with Jekyll front matter ---"
 
           end_index = lines.slice(1..-1).index("---\n")
@@ -50,14 +50,16 @@
         metadata = metadata_for(topic)
         refute_empty metadata, "expected some metadata for topic"
 
-        metadata.each do |key, value|
+        metadata.each_key do |key,|
           assert_includes VALID_METADATA_KEYS, key, "unexpected metadata key '#{key}'"
         end
 
         REQUIRED_METADATA_KEYS.each do |key|
           assert metadata.key?(key), "expected to have '#{key}' defined for topic"
-          assert metadata[key] && metadata[key].strip.size > 0,
-            "expected to have a value for '#{key}'"
+          assert metadata[key]&.strip&.size&.positive?,
+                 "expected to have a value for '#{key}'"
+        end
+      end
         end
       end
     end