`git diff-tree` used to fail on the `3.2` branch (and other non-master
branches) as Travis doesn't actually check that branch from the remote:
```
fatal: ambiguous argument '3.2': unknown revision or path not in the
working tree.
```
The exit code would still be 0 so we'd miss badly formatted commits
targeting stable branches.
We do it manually to ensure that it's going to work as we want it.
(cherry picked from commit e479231b21)
Commits or PRs that do not respect the clang-format style that we
enforce will fail the test.
Adding ubuntu-toolchain-r-test as its libstdc++ is necessary to install
libllvm-3.9.
Rémi Verschelde
Rhody Lugo