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

@@ -10,7 +10,6 @@ group :development do
   gem 'pry'
   gem 'pry-stack_explorer', platforms: %i[ruby_19 ruby_20 ruby_21]
   gem 'rubocop', '0.52.1'
-end
 
   group :test do
     # ActiveSupport required to test compatibility with ActiveSupport Core Extensions.
@@ -23,7 +22,13 @@ group :test do
       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
   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