Issue #295: use a setup_requires dependency

Rather than hand-checking, let setuptools do the checking. This avoids
issues with distutils being unable to parse some setuptools release
versions, which could bite us in future.

setup.py

 #!/usr/bin/env python
-from distutils.version import StrictVersion
 import setuptools
-import sys
-
-
-# Setuptools 17.1 is required, and setup_requires cannot upgrade setuptools
-# in-place, nor trigger the use of a newer version. Abort cleanly up-front.
-setuptools_required = StrictVersion("17.1")
-setuptools_installed = StrictVersion(setuptools.__version__)
-if setuptools_installed < setuptools_required:
-    sys.stderr.write(
-        "mock requires setuptools>=17.1. Aborting installation\n")
-    sys.exit(1)
 
 setuptools.setup(
-    setup_requires=['pbr>=1.3'],
+    setup_requires=['pbr>=1.3', 'setuptools>=17.1'],
     pbr=True)