Snapcraft's remote-build does not fit the requirements of CI,
so replace it with builds running directly on Travis:

1. Builds on Travis can run independently, whereas remote-build
competes for the same resource (Launchpad) and will potentially
block if multiple commits require building in succession.
2. Snapcrafts CLI for remote-build is not designed to be easily
scriptable.
3. Travis recently introduced building on arm64, so builds for
arm64 are now very fast and don't require emulation.

We do not build for armhf and i386 any more because they are
of little relevance on the desktop.