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]
Merge pull request #2 from alphagov/create-release-tag-on-merge-to-master

Add travis config to create release tag on success