Move development dependencies into Gemfile

These should be specified in the Gemfile for development, rather than in
the gemspec.

Author: sds

Gemfile

@@ -1,9 +1,15 @@
 source 'https://rubygems.org'
 
+gemspec
+
+# Development dependencies are listed below
+
+gem 'rake', '>= 10', '< 12'
+gem 'rspec', '~> 3.0'
+gem 'travis', '~> 1.7'
+
 # Generate coverage information in Travis builds
 gem 'coveralls'
 
 # Pin RuboCop for Travis builds.
 gem 'rubocop', '0.39.0'
-
-gemspec

overcommit.gemspec

@@ -28,8 +28,4 @@ Gem::Specification.new do |s|
 
   s.add_dependency             'childprocess', '~> 0.5.8'
   s.add_dependency             'iniparse', '~> 1.4'
-
-  s.add_development_dependency 'rake', '~> 10.4'
-  s.add_development_dependency 'rspec', '~> 3.0'
-  s.add_development_dependency 'travis', '~> 1.7'
 end