Update to trusty, add external repos

Since precise and s3-mirrors wont work (https://github.com/easybib/issues/issues/1967, https://github.com/smira/aptly/issues/98), I upgraded everything to trusty.

Added hhvm.

Using the "ppa" syntax does not work cross-dist, so I enforced external urls and dist trusty.

Removed libboost and docker since they are not available for trusty.