Redirect missing branches to master branch with 301
Redirect missing branches to master branch with 301

- We remove branches once their changes are accepted or rejected so no need to
  serve a 404 if the branch is missing.  We can provide better user experience
  if we redirect to master branch instead.  This helps readers find the guide
  even if they were sent a link with a branch URL.
- Fixes #50.

paulocheque/guides-cms,pluralsight/guides-cms,pluralsight/guides-cms,pluralsight/guides-cms,paulocheque/guides-cms,paulocheque/guides-cms