Variablise RBENV_ROOT from a params class

Since we've referred to it more than once. If you liked it then you shoulda
put a variable on it?
Pass RBENV_ROOT in rbenv::version

Same as applied to `rbenv::rehash` in 89f284f. This probably wont' have been
installing bundler correctly when Puppet was run from cron.

Including some puppetdoc in each class to explain why this is required.
Added tests for the important bits - though it's probably time to split
these out into separate and more descriptive `it{}` blocks.