BF: fix travis builds with new travis setup

Travis now has numpy installed in their python 2 virtualenvs, and it
looks like our own numpy builds were failing as a result.

Don't build numpy for python 2 and use virtualenv numpy instead.
Merge pull request #166 from matthew-brett/fix-travis-builds

BF: fix travis builds with new travis setup

Travis now has numpy installed in their python 2 virtualenvs, and it
looks like our own numpy builds were failing as a result.

Don't build numpy for python 2 and use virtualenv numpy instead.