Travis: skip installing buildifier on Darwin. (census-instrumentation#216)

We don't run format.sh on Darwin in the first place.

Author: g-easy

.travis.yml

@@ -21,13 +21,13 @@ cache:
 
 before_install:
   # Install buildifier if it's not present. It needs at least go 1.8.
-  - if ! which buildifier >/dev/null; then
-      eval "$(gimme 1.11)" ;
-      go get -v github.com/bazelbuild/buildtools/buildifier ;
-    fi
-  # OS X does not have clang-format by default, and has a different sed.
+  # Skip format on OS X: it doesn't have clang-format by default, and has a different sed.
   - if \[ "$TRAVIS_OS_NAME" == "linux" \]; then
-      tools/format.sh ;
+      if ! which buildifier >/dev/null; then
+        eval "$(gimme 1.11)";
+        go get -v github.com/bazelbuild/buildtools/buildifier;
+      fi;
+      tools/format.sh;
     fi
   - wget https://github.com/bazelbuild/bazel/releases/download/0.17.1/bazel-0.17.1-installer-${BAZEL_OS}-x86_64.sh
   - chmod +x bazel-0.17.1-installer-${BAZEL_OS}-x86_64.sh