Move dev dependencies to Gemfile

A similar (unmerged) `rspec-support` PR rspec/rspec-support#517

Reasoning (https://bundler.io/v2.2/man/gemfile.5.html#GEMSPEC):

> The .gemspec file is ... where you specify the dependencies your gem needs *to run*.

Even though it contradicts the very existence of add_development_dependency.

Discussions:
 - rubygems/rubygems#5065
 - rubygems/rubygems#1104

Gemfile

@@ -4,6 +4,13 @@ source 'https://rubygems.org'
 
 gemspec
 
+gem 'rack'
+gem 'rake'
+gem 'rspec', '>= 3.4'
+gem 'rubocop-performance', '~> 1.7'
+gem 'rubocop-rake', '~> 0.6'
+gem 'yard'
+
 local_gemfile = 'Gemfile.local'
 
 if File.exist?(local_gemfile)

rubocop-rspec_rails.gemspec

@@ -38,11 +38,4 @@ Gem::Specification.new do |spec|
   }
 
   spec.add_runtime_dependency 'rubocop', '~> 1.31'
-
-  spec.add_development_dependency 'rack'
-  spec.add_development_dependency 'rake'
-  spec.add_development_dependency 'rspec', '>= 3.4'
-  spec.add_development_dependency 'rubocop-performance', '~> 1.7'
-  spec.add_development_dependency 'rubocop-rake', '~> 0.6'
-  spec.add_development_dependency 'yard'
 end