setup.py: unpin setuptools_scm (#467)

Fixes #465.

Author: blueyed

setup.py

@@ -28,7 +28,7 @@ def read(fname):
     license='BSD-3-Clause',
     packages=['pytest_django'],
     long_description=read('README.rst'),
-    setup_requires=['setuptools_scm==1.11.1'],
+    setup_requires=['setuptools_scm>=1.11.1'],
     install_requires=['pytest>=2.9'],
     classifiers=['Development Status :: 5 - Production/Stable',
                  'Framework :: Django',