Use wheels instead of source packages for builds where possible.

This PR removes `--use-mirrors` from our Travis `pip install` commands (because it provides no real benefit and just makes the installs take longer) and adds `--use-wheel` which will install from a newer packaging format (see http://wheel.readthedocs.org/ for details) which is rather faster for the packages that have wheels available (which sadly isn't very many of them yet).

My test build ( https://travis-ci.org/praekelt/vumi-go/builds/15100414 ) took just over 21 minutes to run compared to the develop build before it ( https://travis-ci.org/praekelt/vumi-go/builds/15045486 ) which took nearly 32 minutes. There's a lot of variance in our build times in general, but this is the quickest successful build in the last few weeks.