.travis.yml: enable testing of other parts of the stack

This patch makes travis test:
 - plainbox (unit tests only), checkbox-support checkbox-ng
 - category, stubbox, checkbox, resource, certification client and
   certification the certification server providers

The checkbox-ng development command is rather odd (different from
everything else) because there seems to be a bug in setuptools when
using scripts=[] and not running setup.py from the current directory.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>