The headers were pointing to the old github wiki. Since we've officially
moved to Confluence, these links should be updated.

If a git version is specified as a branch, and the commit referred to is
already on the local working tree, but the branch is not, then the
checkout will fail.

This is because the Ansible 1.9.3 module would check to see if the
commit was already present, and if so, did not fetch. This meant the
branch wasn't defined in the local working tree.

This can happen if you make a branch but there aren't any changes on the
branch.

Companion PR: https://github.com/edx/edx-analytics-pipeline/pull/204