Pin rake version ~>11.1 to support last_comment

- rspec-rails 3.3.x is pinned by Payola's Rails 4.1 Gemfile
- rspec-rails 3.3.x uses Rake's `last_comment` method
- Rake 12 removes the `last_comment` method (replacing it with
  `last_description`) so rake version < 12 is needed for the Rails 4.1
  Payola build.

An alternative fix would have been to unpin rspec-rails or at least set
it to rspec-rails 3.4.x as rspec-rails 3.4.x does not use Rake's
`last_comment` method. However, the Payola Rails 4.1 build breaks with
rspec-rails 3.4.x as rspec-rails tries to load activejob which is not
available in Rails 4.1. This new issue could be fixed by avoiding
defining the `::ActiveJob` constant unless activejob is supported.
Checking for `::ActiveJob` is how rspec-rails 3.4.x decides if to
load the activejob gem. Currently Payola always defines `::ActiveJob`.

Author: eliotsykes

gemfiles/rails_4_1.gemfile

@@ -10,3 +10,4 @@ end
 gem 'rspec_junit_formatter'
 
 gem 'rails', git: 'https://github.com/rails/rails', branch: '4-1-stable'
+gem 'rake', '~> 11.1'