Detect when binutils are not installed on OSX and skip the test.

Fixes google#342.

.travis.yml

@@ -42,6 +42,9 @@ matrix:
     - os: osx
       osx_image: xcode8.3
       go: master
+    - os: osx
+      env: SKIP_BINUTILS=1
+      go: master
 
 addons:
   apt:
@@ -51,7 +54,7 @@ addons:
 before_install:
   - go get -u github.com/golang/lint/golint honnef.co/go/tools/cmd/...
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update ; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install binutils ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" && -z $SKIP_BINUTILS ]]; then brew install binutils ; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" && -z $SKIP_GRAPHVIZ ]]; then brew install graphviz; fi
 
 script: