BUG: cythonize sources for wheel build

Wheel build was not cythonizing sources, giving a compile error because
needed `numpy/random/mtrand/mtrand.c` was missing.
TST: add travis test for wheel build and install

Add travis build that builds and installs wheel, runs tests from
installed wheel.
Merge pull request #4662 from matthew-brett/cythonize-for-wheels

BUG: cythonize sources for wheel build