Use pre-built versions of our build dependencies on TravisCI

Speeds up our TravisCI build times by 2-4x.

* Old test times:  https://travis-ci.org/enthought/distarray/builds/29751676
* New test times:  https://travis-ci.org/enthought/distarray/builds/29757158
What about putting all these prerequisites in a makefile target or targets?  That would help keep down the amount of stuff going on in .travis.yml and allow for easier testing outside of travis.

The travis command could be just:

```
- if [[ "$TRAVIS_PYTHON_VERSION" == "2.7" ]]; then
    make py2_travis_prereqs
  else
    make py3_travis_prereqs
```