Handle rvm install flags with branch=stable & version=x.y.z.

The readme explains that to use a tagged version that you must set the
branch value to "none" which is overly surprising.

The ultimate solution is cover the method responsible
(#build_script_flags) with unit tests to catch all possible
combinations. Unfortunetly the branch and version flags can interfere
with each other making any solution here subpar.

Closes #152
Output RVM install command to info logger.
Add unit tests for installer flags & integration tests for convergence.
Merge branch 'version-branch-attributes'