chore(.travis.yml): don't test against MRI 1.9.3
chore(.travis.yml): don't test against MRI 1.9.3

We're not using this version of Ruby in any of our applications anymore,
so testing against it is no longer necessary.

kidaa/travis-core,developerfm/travis-core,GistIcon/travis-core,travis-ci/travis-core,jantman/travis-core,Tiger66639/travis-core,GistIcon/travis-core,kidaa/travis-core,Tiger66639/travis-core,final-ci/travis-core,kidaa/travis-core,jantman/travis-core,Tiger66639/travis-core,developerfm/travis-core,travis-ci/travis-core,developerfm/travis-core,travis-ci/travis-core,final-ci/travis-core,GistIcon/travis-core,final-ci/travis-core