Test on 1.9.3 and 2.1.1 & be less wimpy

Our actual "supported" ruby version is 1.9.3 so we should definitely be
testing on that.

It seems excessive to test on an additional *two* future ruby versions. Just
test on 2.1.1 and don't be wimpy about allowing failures. It's currently
passing so let's prep ourselves for a future upversion.

Also three versions can't help our (dreadful) Travis build times...
Merge pull request #390 from alphagov/travis-ruby-versions

Test on 1.9.3 and 2.1.1 & be less wimpy