travis: Disable the OSX build because of the MongoDB requirement
travis: Disable the OSX build because of the MongoDB requirement

MongoDB is not a service provided for the osx build and I do not
want to fight with shell scripts to launch it in the background.

woglinde/osmo-smsc,zecke/osmo-smsc,woglinde/osmo-smsc,woglinde/osmo-smsc,zecke/osmo-smsc,zecke/osmo-smsc