Sets the RVM --binary build_opt based on operatingsystem.

For some OSes there are binaries available for RVM and some, like RHEL,
there are not. Thus, we set the --binary build_opt only for those
with known binaries and build RVM for others.
Merge pull request #3 from ehelms/rvm-binary

Sets the RVM --binary build_opt based on operatingsystem.