~jbicha/firefox/update-dependencies

* New upstream release from the beta channel (FIREFOX_55_0b1_BUILD1)
* Update debian/build/create-tarball.py to create xz-tarballs
* Refresh patches
  - update debian/patches/ubuntu-search-defaults.patch 
  - update debian/patches/ubuntu-ua-string-changes.patch 
  - update debian/patches/unity-menubar.patch
* Remove obsolete --enable-gio option
  - update debian/config/mozconfig.in
* Refresh shipped locales
  - update debian/config/locales.shipped
  - update debian/control
* Don't let debhelper clean Cargo.toml.orig files which are actually required
* Refresh patches
  - update debian/patches/revert-upstream-search-engine-changes.patch
  - update debian/patches/unity-menubar.patch
  - update debian/patches/normalize-distribution-searchplugins.patch