Update requirements.txt
Update requirements.txt

Ran into this error during installation from git:

```Running pystan-2.14.0.0/setup.py -q bdist_egg --dist-dir /var/folders/ky/6sjy1ddn0ml94pyt1n1rbl000000gn/T/easy_install-tfz1yiya/pystan-2.14.0.0/egg-dist-tmp-jjpe_hy7
error: Setup script exited with Cython>=0.22 and NumPy are required.```
jburos/stancache,jburos/stancache