Use new Virtual Environments for Travis

This results in a cleaner build that is less tied to the whims of Travis.  The python 2.7 version uses the system site packages because we are using the Ubuntu versions of Scipy, Matplotlib, and PyQt4.

Well, we *know* the python version, so we could just use `$TRAVIS_PYTHON_VERSION`, right?  I had just copy-pasted the stackoverflow answer.