Consistently use development group in Gemfiles

Using a `development` group isn't really necessary, because all these
Gemfiles are only ever used in the context of development, linting or
running the tests. However, it would be good to be consistent and
using a `development` group probably make things clearer.

Previously we were only using a group in
`gemfiles/Gemfile.minitest.latest` &
`gemfiles/Gemfile.test-unit.latest`.

Closes freerange#706.

Gemfile

@@ -2,16 +2,18 @@ source 'https://rubygems.org'
 
 gemspec
 
-gem 'introspection', '~> 0.0.1'
-gem 'minitest'
-gem 'rake'
+group :development do
+  gem 'introspection', '~> 0.0.1'
+  gem 'minitest'
+  gem 'rake'
 
-if RUBY_ENGINE == 'jruby'
-  # Workaround for https://github.com/jruby/jruby/issues/8488
-  gem 'jar-dependencies', '~> 0.4.1'
-end
+  if RUBY_ENGINE == 'jruby'
+    # Workaround for https://github.com/jruby/jruby/issues/8488
+    gem 'jar-dependencies', '~> 0.4.1'
+  end
 
-if ENV['MOCHA_GENERATE_DOCS']
-  gem 'redcarpet'
-  gem 'yard'
+  if ENV['MOCHA_GENERATE_DOCS']
+    gem 'redcarpet'
+    gem 'yard'
+  end
 end

gemfiles/Gemfile.rubocop

@@ -2,6 +2,8 @@ source 'https://rubygems.org'
 
 gemspec path: '../'
 
-gem 'rake'
-gem 'rubocop'
-gem 'rubocop-rake'
+group :development do
+  gem 'rake'
+  gem 'rubocop'
+  gem 'rubocop-rake'
+end