Update externals to work with Python 3

- Update YAML version to support Python 3
- Python 3 support for ordereddict backport
- Exclude Python3 YAML from version tests.
- Vendor six into Spack.
- Make Python version-check tests work with Python 3
- Add ability to add version check exceptions with '# nopyqver' line
  comments.

.travis.yml

@@ -46,7 +46,6 @@ addons:
     packages:
       - gfortran
       - graphviz
-      - libyaml-dev
 
 # Work around Travis's lack of support for Python on OSX
 before_install: