Shims directory should be prepended to the beginning of the path



In the event that the shims directory is already in the path somewhere but not at the beginning of the path, the "rbenv init -" command does not spit out the 'echo PATH' command at all (by design, per Issue #347). This works for preventing duplication in the path, however it is pretty annoying if you are on a system where RBEnv is being installed globally but it is being added to the path BEFORE places like /usr/bin and /usr/local/bin where global Ruby installations tend to arrive.

A better solution would be to parse the path and actually move the shims directory to the beginning.