Create RVM aliases to allow short Ruby version
Create RVM aliases to allow short Ruby version

These aliases will overrides image's RVM aliases, and allow user to
specify MRI version using `MAJOR.MINOR` version number.

JuliaCI/travis-build,jhass/travis-build,akoeplinger/travis-build,JuliaCI/travis-build,akoeplinger/travis-build,jhass/travis-build,craigcitro/travis-build,andyli/travis-build,andyli/travis-build,craigcitro/travis-build,kevmoo/travis-build,kevmoo/travis-build