add travis condition to only send notifications for master branch

.travis.yml

 services:
   - docker
 
-
+install:
   - pip install PyYAML
   - pip install virtualenv
   - |
     #gem 'kitchen-salt', '>=0.2.25'
   - bundle install
 
+env:
     - PLATFORM=trevorj/salty-whales:trusty
     - PLATFORM=trevorj/salty-whales:xenial
 
+
 before_script:
   - set -o pipefail
   - make test | tail
   - test ! -e .kitchen.yml || bundle exec kitchen test -t tests/integration
 
 notifications:
+  on:
+    branch: master
   webhooks:
     urls:
       - https://webhooks.gitter.im/e/6123573504759330786b