Run git submodule update --force

Overview

In !2600 (merged) we removed testing for Git 1.7 which was the default version of centos 6 which is now reaching the end of life. We used to have a comment that specified we should run git submodule update --force instead but couldn't because we support Git 1.7.

Things to check

1. Make sure untracked files are cleaned out
2. If this should be considered a breaking change because we are no longer depending on 1.7 behavior (which was only shipped in cento6)