Add --no-use-wheel to default pip install arguments

pip 1.4.0 through 1.5.6 (and possibly later versions which haven't come
out, yet) support wheels, but don't upgrade them correctly. So instead
of dealing with virtual environments that can grow cruft, it's better to
just not use wheels for now.

This adds ``--no-use-wheel`` to the default pip install arguments to
prevent pip from using wheels.

Fixes #50
Can we really get away with all these stringwise comparisons? If we ever get to 1.5.10, I could imagine trouble. I bet there's a tuple-ized rendition of the pip version number floating around somewhere.