fixing python2 search

repo2docker/buildpacks/python/__init__.py

@@ -238,10 +238,11 @@ class Python2BuildPack(PythonBuildPack):
     def get_assemble_scripts(self):
         """Return series of build-steps specific to this repository.
         """
+        requirements_file = self.binder_path('requirements.txt')
         return super().get_assemble_scripts() + [
             (
                 '${NB_USER}',
-                'pip2 install --no-cache-dir -r requirements.txt'
+                'pip2 install --no-cache-dir -r "{}"'.format(requirements_file)
             )
         ]