Gemfile: add rubocop 0.59.2 and lock it

Rubocop is a really nice tool when configured properly. A lot of default rules
are very opinionated but the good thing is that it's very easy to disable them.
With help of Rubocop I'd like to improve the quality of Pry's code.

Gemfile

@@ -8,6 +8,11 @@ group :development do
   gem 'yard'
   gem 'rb-inotify', :require => false
   gem 'rb-fsevent', :require => false
+
+  # Rubocop supports only >=2.2.0
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.2.0')
+    gem 'rubocop', '= 0.59.2', :require => false
+  end
 end
 
 group :test do