Skip the simplecov dependency on jruby.

It is not being used and the bigdecimal dependency (added to recent
versions of tins) fails to install on JRuby 9.1.

Gemfile

@@ -10,17 +10,19 @@ end
 group :test do
   gem 'minitest', '~> 5.8'
 
-  # coveralls is no longer maintained, but supports Ruby < 2.3.
-  # coveralls_reborn is maintained, but requires Ruby >= 2.3.
-  gem 'coveralls', git: 'https://github.com/philr/coveralls-ruby.git', require: false if RUBY_VERSION < '2.3'
-  gem 'coveralls_reborn', '~> 0.13', require: false if RUBY_VERSION >= '2.3'
+  unless RUBY_ENGINE == 'jruby'
+    # coveralls is no longer maintained, but supports Ruby < 2.3.
+    # coveralls_reborn is maintained, but requires Ruby >= 2.3.
+    gem 'coveralls', git: 'https://github.com/philr/coveralls-ruby.git', require: false if RUBY_VERSION < '2.3'
+    gem 'coveralls_reborn', '~> 0.13', require: false if RUBY_VERSION >= '2.3'
 
-  # term-ansicolor is a dependency of coveralls. All versions are falsely
-  # declared as compatible with any Ruby version.
-  #
-  # Limit to an earlier compatible version.
-  if RUBY_VERSION < '2.5'
-    gem 'term-ansicolor', '< 1.9'
+    # term-ansicolor is a dependency of coveralls. All versions are falsely
+    # declared as compatible with any Ruby version.
+    #
+    # Limit to an earlier compatible version.
+    if RUBY_VERSION < '2.5'
+      gem 'term-ansicolor', '< 1.9'
+    end
   end
 
   # The source version of ffi 1.15.5 is declared as compatible with Ruby >= 2.3.