Allow rustfmt job to fail if the component is not available

Author: gnzlbg

.travis.yml

@@ -77,8 +77,11 @@ matrix:
       script: cargo test --manifest-path crates/stdsimd-verify/Cargo.toml
       install: true
     - name: "rustfmt"
-      install: rustup component add rustfmt-preview
-      script: cargo fmt --all -- --check
+      install: true
+      script: |
+        if rustup component add rustfmt-preview ; then
+            cargo fmt --all -- --check
+        fi
       rust: stable
     - name: "clippy"
       install: true