Prior to this, the rbenv prompt only showed something if the
$RBENV_VERSION environment variable was set. This was not a complete
solution because rbenv can be configured locally, per directory, with
dotfiles. When using dotfiles, the $RBENV_VERSION variables is not set.
This fixes the issue by taking the output of the `rbenv version-name`
command which shows the real rbenv version being used based on any of
the 4 ways to change rbenv as shown here:
https://github.com/rbenv/rbenv#choosing-the-ruby-version
If the current version of Ruby being used is the same as the global
Ruby, nothing is shown.
This commit also adds documentation to the README that describes the
prompt.
Fixes issue #215
Nate McCurdy