Run make upgrade automatically when running a cookiecutter

[timmc-edx]

The README of each cookiecutter lists make upgrade as a separate post-run setup step. However, this could be done automatically by the post-gen hooks.

• Every layered cookiecutter (that is, all but python-template) includes make upgrade in a post-gen hook
  • Add shared code that all post-gen hooks can use (future: move duplicate stuff from post-gen hooks into shared code area)
  • Shared code should allow creating a virtualenv for running make upgrade in
  • Virtualenv should be deleted by end of setup
• READMEs updated to remove the mention of make upgrade
• Unit tests for all layered cookiecutters check that a fresh virtualenv supports make requirements
• Issues filed for any other steps that can and should be automated