Check whether setup_requires really needs to be defined

[astrofrog]

If so, setup_requires should include Cython if the package includes Cython code and the current version is a developer version. See #131 for a discussion of this.

A better solution might be to simply populate setup_requires in setup.py based on which extensions are being passed to setup.py. I'll work on a PR for that to see if it works. Or we can just check whether Cython files are present.

[cmccully]

Also, numpy often has to be installed before setup if you are using Cython as it often has to tell the c compiler where to find the numpy include files.

[pllim]

Irrelevant due to #438