Add travis config to create release tag on success

- script "borrowed" from stagecraft - we couldnt think of a good way of
  reusing across both repos.
- token generated with gds-pp-ci github user and encrypted using travis
  encrypt command
- tested script works correctly using local testing mode, however we
  wont really know if this has worked until this PR is merged into
  master...

https://www.pivotaltracker.com/story/show/70840424

[Delivers #70840424]