Remove bashisms in bin/clone

pushd/popd are not supported in all /bin/sh implementations, use the
more standard `cd -` to return to initial position. The `cd -` may
actually be completely unnecessary though, as long as the script
is executed rather than sourced the final directory change has no effect.
Merge pull request #1 from ebernhardson/remove-bashism

Remove bashisms in bin/clone