add rake to gemfile for travis

I have been trying to set up the Travis CI service for this project. 
Looking at the ruby specific docs page I see
'Travis CI runs rake by default to execute your tests. Please note that you need to add rake to your Gemfile (adding it to the :test group should be sufficient).'

So that is what I did.
Putting rake in test may be problematic.  The rake gem is a dependency of rails, but is needed outside of just the test environment (example: when you run rake:db migrate it uses this gem).  I think you can remove this.

The test group I think should have the "travis-yaml" stuff, since that's only needed in test environment.