wrap virtualenv around cythonize

bin/cythonize.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+if [ ! -d ".cythonize" ]; then
+    virtualenv .cythonize
+fi
+. .cythonize/bin/activate
+pip install -U -r requirements.txt
+./bin/cythonize.py $@

setup.py

@@ -172,8 +172,7 @@ if not release:
 def generate_cython():
     cwd = os.path.abspath(os.path.dirname(__file__))
     print('Cythonizing sources')
-    p = subprocess.call([sys.executable,
-                         os.path.join(cwd, 'bin', 'cythonize.py'),
+    p = subprocess.call([os.path.join(cwd, 'bin', 'cythonize.sh'),
                          'spacy'],
                          cwd=cwd)
     if p != 0: