Lock rspec to 2.x
* rspec 3 needs capybara > 2.2.0, but capybara-webkit < 2.2.0.
Bundler can't resolve this because rspec doesn't lock the version in the gemspec but throws an exception when run
https://github.com/rspec/rspec-rails/commit/8254ddc10c8e34912300ca2c8df275cc5132d1ba
https://github.com/thoughtbot/capybara-webkit/blob/master/capybara-webkit.gemspec#L22
* Lock 'sass-rails', '~> 4.0.3'