expedite installation on travis

- get wheels from minrk/travis-wheels
- install dependencies with `pip install .[all]`[1], rather than a big explicit `pip install ...` line, which can get out of sync.
- add 'requests' as a dependency in `ipython[test]` because it's needed for several tests

Shaves about 6 minutes off the serial runtime, or 90 seconds per run.

[1] can't actually use `.[all]` syntax for whatever weird pip reason, instead using `file://$PWD#egg=ipython[all]`, but the principal is the same.