.travis.yml: Use wheels[2].astropy.org for Cython

Use astropy wheelhouses for Cython instead of my own. Theirs will probably stay more up to date than mine.

* http://wheels.astropy.org/
* http://wheels2.astropy.org/

wheels2.astropy.org in particular has Linux wheels for Cython 0.20.

Cc: @sontek, @rsyring, @ramiro