add .travis.yml

It's probably a good idea to get this module tested by travis-ci, given the many possible variations here (with or without PadWalker, with or without JSON::XS, new JSON::XS >= 3 or old one, and all the different perl versions...).

With the series of my pull requests everything should be green, see here: https://travis-ci.org/eserte/Data-Dump-Streamer/builds/16112540

Regards, Slaven