support for rbenv

Since I could not set the RBENV_VERSION, even with the command rbenv
shell, I just used the PATH for the specific ruby binary. It works well
for me.

Also I hope I haven't break the rvm :wink2:
updated README
updated gem description
run code for each version in a thread
Merge branch 'support-rbenv'

* support-rbenv:
  updated gem description
  updated README
  support for rbenv