reenable lang mappings in Docker
print out lang mappings during Docker startup
convert lang to sym

lang mappings are sym => string
Revert "convert lang to sym"

unneeded as hashr does this for us

This reverts commit 00d28e01dd5b89abdd49c2bbea48ff1aceec2e08.
correct the docker image selection logic
use the Docker way of setting the cpuset for the container
some hostname for recent docker api changes

Conflicts:
	lib/travis/worker/virtual_machine/docker.rb
Bump travis-build

Includes https://github.com/travis-ci/travis-build/pull/262
Merge branch 'staging'