use rvm for buiding binary ruby

It is basically the same process but moved to rvm where it is handled with other binary rubies using `--movable` flag for compiling ruby.

The binaries build using RVM are published here: https://rvm.io/binaries/osx/10.6/x86_64/ (the same binaries are available for all OSX versions as the static binary is build with `CFLAGS=-mmacosx-version-min=10.6`)