Update highline project dependency to closer match gemspec

* We should probably just use the `gemspec` helper in the bundle so that
  this dependencies Just Happen™ :)
Update dependencies

* Pulls in a few new things, including the newest version of highline
 which is broken, so let's fix that ;)
Explicitly convert inputted email address to string

* Due to an issue in the highline gem, serializing HighLine::Strings
  produce weird results. This is a "fix" for that greater issue.
* https://github.com/JEG2/highline/issues/69
Add ruby 2.0 to ci