Set up Travis CI for soci-pkg

This is a good idea suggested by @kzeslaf in #1 

> Maybe you could also configure building debian packages by Travis CI after
> changes to soci and soci-pkg repos. I don't know unfortunately how to do this nicely.
> Maybe git submodules will do?

I'll try to learn how to configure it.

If anyone knows how to achieve it, please, share the magic.
