drone: remove file

We don't use it.
travis: do not install beanstalkd

We don't use it anymore.
misc: more cleanup

Removing lots of unused stuff. Maybe I'm wrong, let's see what breaks.
docs: remove references to beanstalkd
docs/releases/tsr/0.6.0: beanstalkd is no more
cmd/tsr: add checker for making sure that beanstalkd is not configured

tsr 0.6.0 will refuse to start using beanstalkd, because it doesn't work
anymore :)
cmd/tsr: enable the CheckBeanstalkd in config checkers
etc/tsuru.conf: remove queue-server

I broke my own build :)