~jbicha/firefox/dont-depend-on-gtk2

* New upstream release from the beta channel (FIREFOX_56_0b1_BUILD1)
* Bump build-dep on rustc >= 1.17.0 and cargo >= 1.18
* Build-dep on clang-3.9, llvm-3.9-dev on 64bit archs for stylo
  - update debian/control{,.in}
* Refresh patches
  - update debian/patches/revert-upstream-search-engine-changes.patch
* Drop upstreamed patches
  - debian/patches/build-ppc64.patch
  - debian/patches/fix-no-jit-backend.patch
* Don't try to check checksums of windows-only binary files which are
  not included in the tarball
  - debian/patches/rust-drop-dll-checksums.patch