rbenv-installer needs curl, so mark that as a dependency for ensure_rbenv.
Use 'curl' instead of 'wget' for installing rbenv.
In accordance with the latest instructions at https://github.com/fesplugas/rbenv-installer
Add ensure_user helper
Backticks are evaluated with sh and may not have the rbenv bin path loaded. So, check `which ruby` by using a bash login shell.
Before, we checked met? by seeing if the current rbenv version was the desired version. This doesn't make sense, because simply installing the version doesn't switch us to use that version. Instead, we check `rbenv versions` to see if the desired version has been added to that list.
ruby -v version strings are different in 1.8 and 1.9 unfortunately: ruby 1.8.7 (2011-06-30 patchlevel 352) [i686-linux] ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin11.3.0]