* Update help docs to clarify multiple version(s) are allowed

* Tweek version(s) help description

Otherwise `sed` might not be found later in bats/libexec/bats-exec-test,
in case `/usr/bin` gets removed.

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

Confirms #836

If `foo` didn't exist and `RBENV_VERSION=system rbenv which foo` was
called, the error message used to be misleading:

    rbenv: version `system' is not installed

Instead, have the error message simply say that the command was not found.

Fixes #770

This is useful as an indicator where it is coming from.

This was introduced when merging the rbenv upstream change
(https://github.com/sstephenson/rbenv/commit/6bb7f07d2d).

This is the remaining part of
c69d9a1128.

    commit c69d9a1128
    Author: Mislav Marohnić <mislav.marohnic@gmail.com>
    Date:   Mon Oct 13 12:39:47 2014 +0200

        Isolate rbenv-which tests from any `.ruby-version` file on the system

        Having a `.ruby-version` file in any of the parent directories of the
        local clone of rbenv could cause the test suite to fail because it
        wasn't expecting a local version to be set.

This adds the missing parts from the upstream rbenv merge:
f4652fbbf0

- e4cbf04592
- 3ee395f9b5

Having a `.ruby-version` file in any of the parent directories of the
local clone of rbenv could cause the test suite to fail because it
wasn't expecting a local version to be set.

Fixes #533

This implements removing of the shims path element via bash
substitution, instead of jumping around in all the `$PATH` elements.

Running any shim (and thus `rbenv-exec`) would always execute
`rbenv-version-name` twice: once in `rbenv-exec` and another time in
`rbenv-which`, even though RBENV_VERSION variable would have already
been populated at this point.

Now RBENV_VERSION is respected within `rbenv-which`.

closes #379

It was a dumb idea and it wasn't even implemented perfectly.