diablse sauce labs tests on pull request

* refer <http://docs.travis-ci.com/user/pull-requests/>
* For security reason, travis build for PR can't use
  encrypted data or environment variables. So, tests in
  sauce labs can't run on PR. See details in
  <travis-ci/travis-ci#1946>
* close summernote#1279

.travis.yml

@@ -7,7 +7,7 @@ before_script:
 - npm install -g grunt-cli
 script:
 - grunt test --verbose
-- grunt saucelabs-test
+- '[ "${TRAVIS_PULL_REQUEST}" = "false" ] && grunt saucelabs-test
 - grunt karma:travis
 - grunt coveralls:travis
 before_deploy: