Minimal state/yaml validation

This adds a minimal validation test by:

* Installing salt
* Copying the states to a preconfigured spot (`/srv/salt/states`), as specified by `.travis/minion`
* Running `sudo salt-call state.show_lowstate --local --retcode-passthrough`

We could add more, but I figured I'd give out this minimal setup and get your take on it. No badge is added to the README in this PR (currently).