MAINT: give up on rackspace wheels

Rackspace wheels (from http://wheels.scikit-image.org) are giving
timeouts, again - e.g.:

https://travis-ci.org/numpy/numpy/jobs/40578751

Use Nathaniel's trick of not compiling Cython, for speed, and to work
round previous Cython build breakage.
Merge pull request #5271 from matthew-brett/give-up-on-rackspace-wheels

MAINT: give up on rackspace wheels