When available, patch defaults to latest stable

This is just a small enhancement to enable the user to only state the required version, using the latest stable patch by default.

For example, `rbenv install 1.9.3` has the same effect as `rbenv install 1.9.3-p545`.

Works on OS X Mavericks. Did not create any bats test for this as I did not find a tests file for CLI details (therefore, assumed it would not be required).