travis: add required nightly toolchain and wasm-gc

.travis.yml

@@ -10,6 +10,8 @@ rust:
 
 before_script:
   - rustup component add rustfmt --toolchain 1.40.0-x86_64-unknown-linux-gnu
+  - rustup update nightly
+  - cargo +nightly install --git https://github.com/alexcrichton/wasm-gc --force
 
 script:
   # Ensure all checked in code is cargo-fmt'ed