c library update script: use new mavgen wrapper

The new wrapper was introduced in
8eb6635a4e24c48a3660e03be5817fb815c9e3f3 and thus mavgen can not be
called directly anymore.
travis: use new mavgen wrapper
make tools/mavgen.py executable
travis: set pythonpath
gen sh scripts: use mavgen.py from tools directory
Merge pull request #297 from mavlink/clibscriptfix

c library update script & travis: use new mavgen wrapper