Lock rubocop version to the minor series

Since `rubocop` is pre-1.0, breaking changes are often introduced with
each minor version release. By locking to a minor version series, we
can update to the latest on our own schedule and avoid unexpected build
failures in the meantime.

Gemfile

@@ -26,7 +26,7 @@ group :test do
     gem "cucumber", "~> 2.0"
     gem "phantomjs", "~> 1.9"
     gem "poltergeist", "~> 1.1"
-    gem "rubocop", ">= 0.30"
+    gem "rubocop", "~> 0.36.0"
     gem "test-unit", "~> 3.0"
   end
 end