Add checks for version 2.7

bin/spack

@@ -1,7 +1,9 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
+import sys
+if not sys.version_info[:2] >= (2,7):
+    sys.exit("Spack requires Python 2.7.  Version was %s." % sys.version_info)
 
 import os
-import sys
 import argparse
 
 # Find spack's location and its prefix.