install yarn on osx for travisci

Signed-off-by: Nassim 'Nass' Eddequiouaq <[email protected]>

Author: n4ss

.travis.yml

@@ -3,13 +3,14 @@
   sudo: required
   language: go
   dist: trusty
+  osx_image: xcode9
   os:
     - linux
     - osx
   notifications:
     email: false
   go:
-    - 1.8
+    - 1.8.1
   before_install:
     # work-around for issue https://github.com/travis-ci/travis-ci/issues/6307
     # might not be necessary in the future