BLD: require more recent cython

[jreback]

see comments here

we currently require >= 0.19.1. So probably should move that up.

Not super pressing though.

[jreback]

cc @wesm

[jreback]

should we suppress this warning

(pandas) [Thu Oct 27 06:45:37 ~/pandas]$ make
python setup.py build_ext --inplace
/Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/Cython/Distutils/old_build_ext.py:29: UserWarning: Cython.Distutils.old_build_ext does not properly handle dependencies and is deprecated.
  "Cython.Distutils.old_build_ext does not properly handle dependencies "
running build_ext
cythoning pandas/hashtable.pyx to pandas/hashtable.c
building 'pandas.hashtable' extension
creating build
creating build/temp.macosx-10.6-x86_64-3.5
creating build/temp.macosx-10.6-x86_64-3.5/pandas
...