Added @ApacheMesos release tweet to the release guide.
Added @ApacheMesos release tweet to the release guide.

We forgot to tweet the 1.8.0 and 1.9.0 releases, so this adds
it to the release guide.
dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos