Update .kitchen.yml to provsionless vagrant base boxes.
Update suite test Ruby version.
NO MORE RDOC WHEN COMPILING RUBIES IN TEST!

Seriously, this burns up so much time in the build process. Some hints
taken from sstephenson/ruby-build#156.
Drop default "global" version in rbenv_script provider.

This will lead to errors if rbenv_script is invoked before any rbenv
Ruby versions are installed (and a global Ruby version is set).

References #33
References #43