Remove Rake dependency

We never used the `bundler/gem_tasks` when cutting a release, so remove
this dependency on the rake gem.

This saves us from having to cut a new release every time a new major
Rake version is released in order to keep our Gemnasium badge happy.

Author: sds

Gemfile

@@ -4,7 +4,6 @@ gemspec
 
 # Development dependencies are listed below
 
-gem 'rake', '>= 10', '< 12'
 gem 'rspec', '~> 3.0'
 
 # Generate coverage information in Travis builds