Deployment User Setup

The `continuous_integration.rb` now uses the Travis CLI tool to generate SSH keys for the deployment user.
@vandahm what do you think of the approach of:
* installing the latest version from rubygems
* invoking `travis -v` to get the version number of the installed gem
* using that version and writing it into the Gemfile.

see commit: 1a8f3979e6160be39281c4bdb0160c2adff74690