Add ability to prefer particular versions in packages.

- Adding `preferred=True` to a version directive will change its sort
  order in concretization.

- This provides us a rudimentary ability to keep the Spack stack
  stable as new versions are added.

- Having multiple stacks will come next, but this at least allows us
  to specify default versions of things instead of always taking the
  newest.

var/spack/packages/python/package.py

@@ -17,7 +17,7 @@ class Python(Package):
 
     version('2.7.8', 'd235bdfa75b8396942e360a70487ee00')
     version('2.7.10', 'c685ef0b8e9f27b5e3db5db12b268ac6')
-    version('2.7.11', '1dbcc848b4cd8399a8199d000f9f823c')
+    version('2.7.11', '1dbcc848b4cd8399a8199d000f9f823c', preferred=True)
     version('3.5.0', 'd149d2812f10cbe04c042232e7964171')
     version('3.5.1', 'e9ea6f2623fffcdd871b7b19113fde80')