linux: Fix compilation warning from dbus
Link with dbus
linux: Fix detect global menu bar

It seems that detecting the bus name "com.canonical.AppMenu.Registrar"
is the most reliable way to detect whether the appmenu is available, see
http://git.io/JmP7Yg for the discussion.
Upgrade libchromiumcontent for dbus headers
linux: Add libdbus-1-dev as requirement
Merge pull request #726 from atom/dbus-checking-menu

Fix detecting global menu bar on Ubuntu