Since we only use sudo for installing packages, start using the apt
travis addon for the installation in order to allow container-based
builds to be used for testing dunst which should theoretically have a
faster boot time.
Change the behaviour of travis to only notify if the build status
changes. The previous behaviour which was notifying for every build
regardless of status added a lot of unnecessary noise to the IRC
channel.
Remove xext and xft as a dependency since they are not currently used
and probably haven't been used a while.
Also remove freetype2 from the pkg-config list, while it is required by
pangocairo it should be included from the pkg-config --cflags call.
