require cython for installation of postpic

setup.py

@@ -37,7 +37,8 @@ setup(name='postpic',
       include_dirs = [numpy.get_include()],
       license='GPLv3+',
       setup_requires=['cython>=0.18'],
-      install_requires=['matplotlib', 'numpy>=1.7', 'scipy', 'future', 'urllib3', 'numexpr'],
+      install_requires=['matplotlib', 'numpy>=1.8', 'scipy', 'future', 'urllib3', 'numexpr',
+                        'cython>=0.18'],
       extras_require = {
         'h5 reader for openPMD support':  ['h5py'],
         'sdf support for EPOCH reader':  ['sdf']},