Move RuboCop version pinning into Gemfile

We were declaring the pinned version of RuboCop as a
development dependency in the gemspec. This turns out to not be
technically correct, as it's not really a development dependency, but a
testing one (i.e. we don't want to enforce this requirement for
developers using this gem when developing other gems that require this
gem).

It turns out a way get around this is to declare the pinned version in
the `Gemfile`.

Change-Id: Ifa27a9c2cec89297300fc99f0aae483839eb636c
Reviewed-on: http://gerrit.causes.com/44724
Tested-by: jenkins <[email protected]>
Reviewed-by: Shane da Silva <[email protected]>

Gemfile

@@ -1,3 +1,10 @@
 source 'http://rubygems.org'
 
 gemspec
+
+# Pin Rubocop for Travis builds.
+# This won't affect people installing via `gem install` since it's not in the
+# gemspec. It needs to come before the call to `gemspec` below in order to
+# prevent a warning being displayed about duplicate dependencies when running
+# `bundle install` in this repository.
+gem 'rubocop', '0.27.1'

scss-lint.gemspec

@@ -29,5 +29,4 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'nokogiri', '~> 1.6.0'
   s.add_development_dependency 'rspec', '~> 3.0'
-  s.add_development_dependency 'rubocop', '0.27.1'
 end