Update .travis.yml

.travis.yml

@@ -11,9 +11,10 @@ os:
   - linux
   - osx
 sudo: true
-before_install:
+install:
-  - sudo apt-get -qq update
+  - if [ $TRAVIS_OS_NAME = linux ]; then sudo apt-get -qq update; fi
-  - sudo apt-get install -y xclip
+  - if [ $TRAVIS_OS_NAME = linux ]; then sudo apt-get install -y xclip; fi
+  - if [ $TRAVIS_OS_NAME = osx ]; then sudo brew install xclip; fi
 
 # postrun
 after_success: