Only 'cythonize' if Cython is installed.

setup.py

@@ -1,11 +1,18 @@
 from setuptools import setup
 from setuptools import find_packages
 from distutils.extension import Extension
-from Cython.Build import cythonize
+
+try:
+    from Cython.Build import cythonize
+except ImportError:
+    def cythonize(extensions): return extensions
+    sources = ['rocksdb/_rocksdb.cpp']
+else:
+    sources = ['rocksdb/_rocksdb.pyx']
 
 mod1 = Extension(
     'rocksdb._rocksdb',
-    ['rocksdb/_rocksdb.pyx'],
+    sources,
     extra_compile_args=[
         '-std=c++11',
         '-O3',
@@ -32,10 +39,7 @@ setup(
     author_email="Use the github issues",
     url="https://github.com/stephan-hof/pyrocksdb",
     license='BSD License',
-    install_requires=[
-        'setuptools',
-        'Cython>=0.20',
-    ],
+    install_requires=['setuptools'],
     package_dir={'rocksdb': 'rocksdb'},
     packages=find_packages('.'),
     ext_modules=cythonize([mod1]),