Fix typo in .travis.yml.

I found this typo while I was unashamedly copying your Travis-CI file. I don't think it was making a difference because `$PYTHONPATH` (and certainly `$PYTONPATH`) are probably unset anyway. But I figured I'd fix it just the same.

RPLY and Topaz are great projects, by the way. I've learned a lot about RPython from both of them!