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.

Not sure this is much better, but how about

```
python -c "import sys; v = sys.version_info; print('%s.%s' % (v.major, v.minor))"
```

or

```
python -c "import sys; v = sys.version_info; print('{0}.{1}'.format(v.major, v.minor))"
```