Update travis config to current ruby versions.
Update travis config to current ruby versions.

This has the nice side effect of fixing the failing travis builds. :-)
The build failures seen with older ruby versions (for example on 1.9.3) seem to be due to the following bundler bug:
https://github.com/bundler/bundler/issues/3558

ibetdogg/test,Shopify/dashing,Shopify/dashing,ibetdogg/test,elfinith/racedata-dashboard,ematthews/dashing,ibetdogg/test,elfinith/racedata-dashboard,ematthews/dashing,ematthews/dashing,elfinith/racedata-dashboard,elfinith/racedata-dashboard,Shopify/dashing