allow multiple locations for the global rbenv version file.

closes #2

By default rbenv.el will check for the global version file in the following locations:
* ~/.rbenv/version
* /usr/local/rbenv

You can modify the list `rbenv-global-version-files` to change the locations where rbenv.el
will look for your global version file.
Merge pull request #3 from senny/multiple_global_version_locations

allow multiple locations for the global rbenv version file.