travis: add libcap2-bin package to have setcap.

If travis can install this package, it would allow it to perform testing
with the minimum required capabilities.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
travis: run tests without elevated priviledges.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
Merge pull request #16 from eroullit/travis-setcap

Add libcap2-bin package to Travis to support setcap.