Modify setup.py so pip install completes succesfully (#231)

Including pystan & Cython in the setup_requires caused
pip install to fail with a complaint about missing
Cython and numpy dependencies.  Removing the explicit
Cython/numpy dependencies allows pip install to
complete successfully (pystan pulls in Cython/numpy).

python/setup.py

@@ -104,12 +104,9 @@ setup(
     license='BSD',
     packages=['fbprophet', 'fbprophet.tests'],
     setup_requires=[
-        'Cython>=0.22',
-        'pystan>=2.14',
     ],
     install_requires=[
         'matplotlib',
-        'numpy',
         'pandas>=0.18.1',
         'pystan>=2.14',
     ],