Migrate to rustfmt-preview and require rustfmt builds to pass (rust-lang#353)

* migrate to rustfmt-preview and require rustfmt to pass

* reformat with rustfmt-preview

Author: gnzlbg

.travis.yml

@@ -27,15 +27,15 @@ matrix:
     - script: cargo test --manifest-path crates/stdsimd-verify/Cargo.toml
       install: true
     - env: RUSTFMT=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
-      script: |
-        cargo install rustfmt-nightly --force
-        cargo fmt --all -- --write-mode=diff
+      before_script:
+      - rustup component add rustfmt-preview
+      script:
+      - cargo fmt --all -- --write-mode=diff
     - env: CLIPPY=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
       script: |
         cargo install clippy
         cargo clippy --all -- -D clippy-pedantic
   allow_failures:
-    - env: RUSTFMT=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
     - env: CLIPPY=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
 
 before_install: