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

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.

test/which.bats

@@ -96,7 +96,7 @@ echo HELLO="\$(printf ":%s" "\${hellos[@]}")"
 exit
 SH
 
-  PYENV_HOOK_PATH="$hook_path" IFS=$' \t\n' run pyenv-which anything
+  PYENV_HOOK_PATH="$hook_path" IFS=$' \t\n' PYENV_VERSION=system run pyenv-which anything
   assert_success
   assert_output "HELLO=:hello:ugly:world:again"
 }