Add basic Travis CI configuration.

[AlekSi]

Refs #14.

[umputun]

thx for the PR. two questions:

1. why sudo: required ?
2. dist: trusty - probably doesn't matter as we use docker build, but still

edit: I guess docker needs sudo on travis. Is this the usual way to run docker build
or they have a docker-friendly way?

[AlekSi]

This page states that both sudo: required and dist: trusty are not in fact required – the presence of docker commands in install and script is enough. See #46 for example.
But I would keep configuration as it is now. This and this pages both state that those lines are required. Otherwise, the build could be routed to container-based infrastructure. I think those pages are just outdated. But Travis CI does change defaults and routing rules from time to time, so I would keep things explicit as they are now.

Is this the usual way to run docker build or they have a docker-friendly way?

That's the recommended way. They optimize for building with a set of bash commands. If it does not requires Docker (or explicitly requires full VM for some reason) they have a flexibility of routing a build to a less loaded infrastructure.