Pin Rubocop in Gemfile instead of gemspec

We don't want to declare this as a development dependency because it's
not really a dependency--it's simply a tool we run against the
repository as part of our CI runs.

Change-Id: Ica1bdbe5de39ee88f8e114a7cbce4cc6e3937844
Reviewed-on: http://gerrit.causes.com/45031
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

Gemfile

@@ -1,3 +1,6 @@
 source 'http://rubygems.org'
 
+# Pin Rubocop for Travis builds.
+gem 'rubocop', '0.26.0'
+
 gemspec

overcommit.gemspec

@@ -30,5 +30,4 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'image_optim', '~> 0.15.0'
   s.add_development_dependency 'rspec', '~> 3.0'
-  s.add_development_dependency 'rubocop', '0.26.0' # Pin for Travis builds
 end