Don't use rustfmt on CI

`rustfmt` hasn't been available on a number of nightlies now, and so it is
disrupting our testing of new PRs. I'm also reconsidering whether it's a
good idea to block PRs landing on formatting, and so I'm removing this
check on CI.

.travis.yml

@@ -23,11 +23,7 @@ branches:
 before_install:
   - sudo apt-get install -y acpica-tools
 
-before_script:
-  - rustup component add rustfmt-preview
-
 script:
-  - cargo fmt --all -- --check
   - cargo build --all
   - cargo test --all
   - cargo run --bin aml_tester -- -p tests