setup git committer info in Travis

pkg/ Makefiles based on `Makefile.git` fail in Travis when attempting to apply patches with `git am` because no committer info is set.
This patch adds a dummy Travis username and mail in the 'install' step to satisfy git.

[Travis log](https://travis-ci.org/RIOT-OS/RIOT/builds/29360429#L1056) of the problematic behavior