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>
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