Update build doc to meet Cython requirement in setup.py

Author: Alex Hagerman

README_building_and_developing.rst

@@ -10,7 +10,7 @@ To build pymssql you should have:
 * python >= 2.7 including development files. Please research your OS usual
   software distribution channels, e.g, ``python-dev`` or ``python-devel``
   packages.
-* Cython >= 0.15
+* Cython >= 0.19.1
 * FreeTDS >= 0.91 including development files. Please research your OS usual
   software distribution channels, e.g, ``freetds-dev`` or ``freetds-devel``
   packages.