ci: build x86_64-pc-windows-gnu without containerisation

As of 2016-09-21 Travis started serving Ubuntu precise containers when
trusty was requested. This caused the x86_64-pc-windows-gnu build to
break, but we only noticed when building on custom Travis branches
because the geckodriver master branch used a cached Rust installation.

This change reverts the 64-bit Windows builds to run outside of the
container infrastructure, in which we can guarantee we are served Ubuntu
trusty with a sufficiently modern gcc to cross-compile.

.travis.yml

@@ -36,6 +36,7 @@ matrix:
         - NAME=win64
         - EXT=zip
       dist: trusty
+      sudo: required
       addons:
         apt:
           packages: