Depend on cython 0.20 to use the @cython.no_gc_clear decorator

docs/installation.rst

@@ -33,5 +33,5 @@ Building pyrocksdb
     $ virtualenv pyrocks_test
     $ cd pyrocks_test
     $ . bin/active
-    $ pip install Cython
+    $ pip install "Cython>=0.20"
     $ pip install git+git://github.com/stephan-hof/pyrocksdb.git

setup.py

@@ -29,7 +29,7 @@ setup(
     name="pyrocksdb",
     install_requires=[
         'setuptools',
-        'Cython',
+        'Cython>=0.20',
     ],
     package_dir={'rocksdb': 'rocksdb'},
     packages=find_packages('.'),