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.
it isn't important but I have been thinking on switching our supervisor based images to http://phusion.github.io/baseimage-docker/, it offers nice features an was recently upgraded to ubuntu trusty too.