Add cython to install_requires (#232)

Author: nchristensen

setup.py

@@ -323,7 +323,7 @@ def run(self):
             'charmrun = charmrun.start:start',
         ],
     },
-    install_requires=['numpy>=1.10.0', 'greenlet'],
+    install_requires=['numpy>=1.10.0', 'greenlet', 'cython'],
     #python_requires='>=2.7, ~=3.4',
     classifiers=[
         'Intended Audience :: Developers',