Fix env handling

Source `rbenv-plugin-env` instead of trying to source `rbenv-path-env`,
which is not present in the `rbenv-plugin` install (and would introduce
a circular reference even if correctly pointing to `rbenv-path`, as the
recommended way to install `rbenv-path` is ... `rbenv-plugin`).

No replacement for `rbenv-path-shared` needed, as the functions defined
in that file are not used anywhere in the code base.
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.