Allow installing rustfmt component to fail

.travis.yml

@@ -23,11 +23,14 @@ matrix:
         - shellcheck ci/*.sh
       stage: tools-and-build-and-tier1
     - name: "Style"
-      install: rustup component add rustfmt-preview
+      install: true
       script:
         - rustc ci/style.rs && ./style src
         # Disabled due to rust-lang/rustfmt#3341
-        # - cargo fmt --all -- --check
+        #- |
+        #  if rustup component add rustfmt-preview ; then
+        #      cargo fmt --all -- --check
+        #  fi
       stage: tools-and-build-and-tier1
 
     # BUILD stable, beta, nightly