* firefox-beta-bin: 51.0b8 -> 54.0b13
* firefox-devedition-bin: init at 54.0b14
Firefox DevEdition became a new product of Mozilla and is "repackaged"
Firefox Beta with its own release channel and six weeks release cycle as
other channels. It is no longer being built on nightly basis
* updated the update.nix script to facilitata firefox-devedition-bin
* disabling automatic updates by pointing to non existing channel
* f firefoxWrapper looks for gtk3 attribute to wrap the executable gtk3 to wrap the binary with needed ``XDG_DATA_DIRS``
* removed firefox-developer-bin
* extend firefox-bin expression to accept sources as input so
firefox-developer-bin can be packaged outside nixpkgs
* generate_sources.rb now includes full urls in the output
