clang-format versioning

It might be a good idea to force a particular version of clang-format as they produce different output. For example, we can force install clang-format 6 and then use that. CS50 Sandbox seems to use the earliest version of clang-format among platforms used in CSUF CPSC 121 (CS50, Tuffix 2019, Tuffix 2020). It uses clang-format version 6 out of ubuntu 18.04