Maint: change default ruby version to 'default'

I don't really care exactly what version this is using.  I want to use
rvm more for it's gemfiles.  RVM supports calling the version 'default'.
I don't know if other ruby environment managers support that though. I
know passenger doesn't if someone comes across this commit.

Longterm, may consider dropping the rvm config files from project and
leave that as an exercise to the user to setup how they want.