Fix rbenv-plugin-env killing commands

The `rbenv-path-add` guard tries to append to the non-existant
`$RBENV_ROOT/bin` dir and returns 1 when that predictably fails.
With `set -e`, this aborts the script.

Personally, I file this under “set -e considered harmful” as I side with
http://mywiki.wooledge.org/BashFAQ/105, but removing the `rbenv-path`
guard seems to be the path of least resistance.