Add support for removing versions.

Add an ensure parameter to rbenv::version.  When set to 'absent', this
will remove the version from the system.
Disable 80char lint check.

Complying with this check would require making the code less readable.
It's only been triggered because a block has been indented due to an
extra conditional, and a line assigning a string to a variable is now 82
chars long.
Merge pull request #15 from gds-operations/version_absence

Add support for removing versions.