Switch to only setting Bundler as a dev dependency

Our contributing documentation specifically mentions Bundler so we
should set it as a development dependency.

Gemfile

@@ -10,20 +10,25 @@ group :development do
   gem 'pry'
   gem 'pry-stack_explorer', platforms: %i[ruby_19 ruby_20 ruby_21]
   gem 'rubocop', '0.52.1'
+
+  group :test do
+    # ActiveSupport required to test compatibility with ActiveSupport Core Extensions.
+    # rubocop:disable Bundler/DuplicatedGem
+    require File.expand_path('../lib/hashie/extensions/ruby_version', __FILE__)
+    if Hashie::Extensions::RubyVersion.new(RUBY_VERSION) >=
+       Hashie::Extensions::RubyVersion.new('2.4.0')
+      gem 'activesupport', '~> 5.x', require: false
+    else
+      gem 'activesupport', '~> 4.x', require: false
+    end
+    # rubocop:enable Bundler/DuplicatedGem
+    gem 'rake'
+    gem 'rspec', '~> 3'
+    gem 'rspec-pending_for', '~> 0.1'
+  end
 end
 
 group :test do
-  # ActiveSupport required to test compatibility with ActiveSupport Core Extensions.
-  # rubocop:disable Bundler/DuplicatedGem
-  require File.expand_path('../lib/hashie/extensions/ruby_version', __FILE__)
-  if Hashie::Extensions::RubyVersion.new(RUBY_VERSION) >=
-     Hashie::Extensions::RubyVersion.new('2.4.0')
-    gem 'activesupport', '~> 5.x', require: false
-  else
-    gem 'activesupport', '~> 4.x', require: false
-  end
-  # rubocop:enable Bundler/DuplicatedGem
   gem 'codeclimate-test-reporter', '~> 1.0', require: false
   gem 'danger-changelog', '~> 0.1.0', require: false
-  gem 'rspec-core', '~> 3.1.7'
 end

hashie.gemspec

@@ -24,7 +24,5 @@ Gem::Specification.new do |gem|
     }
   end
 
-  gem.add_development_dependency 'rake', '< 11'
-  gem.add_development_dependency 'rspec', '~> 3.0'
-  gem.add_development_dependency 'rspec-pending_for', '~> 0.1'
+  gem.add_development_dependency 'bundler'
 end