Reconfigure travis CI to use docker images for easier replication

[razzmatazz]

There was/is one PR where CI failure could not be replicated on some of the platforms

• https://github.com/OmniSharp/omnisharp-emacs/pull/281

Using docker for CI/test builds would make them easier to replicate AND probably faster to run too.

This seems to be the latest/most updated docker hub repo for emacs:

• https://hub.docker.com/r/jare/emacs/