updated Dockerfile

* switch to latest Ubuntu LTS;
* install adblockparser, re2 and pyre2

//cc @dangra

Also, maybe add docs on running Splash via Docker? It'd be great to explain how to e.g. add custom configs to the image or change Splash options. This may be a way to fix https://github.com/scrapinghub/splash/issues/71.
"ADD" directive will invalidate the caching of "RUN pip install" call, it is good practice to install pinned dependencies before "ADD . /app" because of this.