rbenv plugin: don't clobber existing RBENV_ROOT & follow Homebrew's default behavior

Currently the rbenv plugin clobbers any existing RBENV_ROOT env var and always sets $RBENV_ROOT to the directory containing rbenv's bin/ subdir. This isn't always what you want.

For example, when rbenv is installed via Homebrew on OS X, it leaves RBENV_ROOT set to `~/.rbenv` so that Rubies will be installed there, but the bin/ dir remains in `/usr/local/[blahblah]`. This works just fine, and in fact some programs (such as JetBrains' RubyMine) only support rbenv with Rubies installed in `~/.rbenv`, so it's important to leave it there when possible.

This PR leaves existing RBENV_ROOT values intact and causes Homebrew-installed rbenv's to default to the same setting that Homebrew would (`~/.rbenv`).