Merge rbenv's "Isolate rbenv-which tests …" (c69d9a11)

This is the remaining part of
https://github.com/sstephenson/rbenv/commit/c69d9a1128283f20ad883178e3649d7ed92be663.

    commit c69d9a1128283f20ad883178e3649d7ed92be663
    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.