Change travis to use wheel binaries

This was discussed at length in #938 .  The basic idea is that our travis tests run slowly for a variety of reasons, but one of the important ones is that it has to build everything from source.  This PR changes the travis behavior to instead install numpy, scipy, and Cython from binaries (in the new "wheel" format - see [PEP427](http://www.python.org/dev/peps/pep-0427/)).

Once the tests on this run we'll see what the speedup is exactly, but generally I've been getting < 1 hr total for all of the tests on this, vs ~2.5 hr for the current master branch.  Also, as a bonus, the build/test log for astropy is now *much* simpler, because it doesn't have all the crap from scipy and numpy getting built.  

This is also enhanced from previous versions discussed in #938 in that it will fall back on the build-from-source methodology if the wheel fails to install for whatever reason.  That will of course slow everything down, but at least they don't just all fail if there's a web server hiccup or something.

A few items remain before this can get merged, though:

- [ ] Currently the wheels are hosted on a web server that was not really intended for file downloads, so this shouldn't get merged until we find something somewhat more suitalbe.  I believe @astrofrog volunteered to use his jenkins mac mini for this purpose until we find a more permanent replacement - @astrofrog, can you let me know when you get the files I sent you on that server (and point me to the right URL so I can update this)? 
- [ ] This version has open file detection turned *off*.  I ran two builds - one where it was [on](https://travis-ci.org/eteq/astropy/builds/6651613) and one where it was [off](https://travis-ci.org/eteq/astropy/builds/6651637).  This shows that now unclosed file detection works on all versions... but it also slows down the build process by almost a factor of two.  So I suggest we turn on open file detection on jenkins, but leave it off for travis.  Does that sound ok, @mdboom ?

(cc @iguananaut @astrofrog @keflavich as interested parties from #938)