Test if it compiles for redox too

.travis.yml

@@ -2,6 +2,8 @@ language: rust
 cache: cargo
 before_script:
 - rustup component add rustfmt-preview
+- rustup target add x86_64-unknown-redox
 script:
 - bash ./fmt.sh -- --write-mode=diff
 - bash ./test.sh
+- cargo build --target=x86_64-unknown-redox