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

Using `hitch` with Ruby 2.0 produces weird values for the "group email". This is due to a serialization issue with highline which appears to trickle all the way down to Ruby's YAML parser psych (see the above referenced issue).

See https://gist.github.com/iamvery/7218658 for example "bad" serialization.