Fix the end-to-end Signonotron integration tests.

* Bump mocha and rspec-rails to mutually compatible versions which
  work with newer versions of Rails.
* Update test helper 'require' to remove deprecation warning.
* Remove the Bundler.require for development dependencies, as it was
  trying to require 'mocha' directly, which is now deprecated.
* Add in explicit requirements for development dependencies.
Merge pull request #14 from alphagov/fix-integration-tests

Fix the end-to-end Signonotron integration tests