Added capybara gem + updated spec_helper to use it

**This pull requests adds the capybara gem, used to simplify integration testing, to the gemset of this project.
The `spec_helper.rb` file has been edited, so capybara will be included in all specs automatically.**

Note that this probably concerns all dev teams, as every team will have to write integration tests. Therefore i suggest this to be merged into the general develop branch.  
Capybara documentation can be found [here](http://rubydoc.info/github/jnicklas/capybara/master).

_Don't forget to do a `bundle install` after pulling this change._