Provide support for building tests on travis

If you're interested, this will build tests automatically on Travis, but you'll need enable the hooks for this repository.

A change to `nksip_counters` was necessary because the build was failing on R16 was failing due to the gen_server already being started.  For reference, the failed build is at: https://travis-ci.org/danielwhite/nksip/jobs/10390534.