travis: Move cloudabi to xbuild section

Cloud ABI [is now Tier 3](https://github.com/rust-lang/rust/pull/75568),
so its libstd is no longer distributed with Rust.

Firefox testing is also disabled due to:
  https://github.com/rustwasm/wasm-bindgen/issues/2261



Signed-off-by: Joe Richey <joerichey@google.com>