Update clang-format CI to track newer LLVM
The clang-format tests should track a version of clang-format the ICHEC developers can easily install on their laptops.
clang-format on Ubuntu 18.04 and the latest clang-format on homebrew (LLVM 8.0, or so) produce different results.
Using a newer Ubuntu image for this test might fix it.
Edited by Pádraig Ó Conbhuí