Merge pull request ewasm#71 from ewasm/circleci

Simplify circleci around rustfmt

Author: axic

circle.yml

@@ -10,14 +10,13 @@ jobs:
           name: Update rustc
           command: |
             rustup target add wasm32-unknown-unknown
-            rustup component add rustfmt-preview
+            rustup component add rustfmt
             rustup update
       - run:
            name: Check formatting
            command: |
              rustfmt --version
-             cargo fmt
-             git diff --exit-code
+             cargo fmt --all -- --check
       - run:
           name: Test
           command: cargo test --target=x86_64-unknown-linux-gnu