Dont run browser tests in PRs.

As secure $ENVs are disabled for pull requests and those $ENVs are required for running browser tests on saucelabs.

http://docs.travis-ci.com/user/pull-requests/#Security-Restrictions-when-testing-Pull-Requests

+ updated readme with the fact that browser tests are only runned automatically when *merged into master*, not in the PR stage

.travis.yml

@@ -7,7 +7,7 @@ before_install:
   - npm install -g npm
 script:
   - npm test
-  - "test $TRAVIS_NODE_VERSION != '0.12' || npm run test-browser"
+  - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '0.12' || npm run test-browser"
 env:
   global:
   - secure: L0dg0jr2fwkc2tPwP5svybILPBn2qdLzMrWc5tEXg3MPcy8D59Gvf+ri7INqo+ETPM20o5CsaDCH+LHUNS/V0G4VG1ajvsy7d8uh3hnb/K6VfVui/CjsHIqOcOZrbxVxgyX+iMXEXAj0+Syow9uDQHVhrz1qqad1n79likNCXa4=

README.md

@@ -524,7 +524,7 @@ Then, you can run the test with:
 
 ### Browser tests
 
-Browser tests are not included in `npm test` as they run for too long, although they are runned automatically on Travis upon commit. Run browser tests locally in any browser:
+Browser tests are not included in `npm test` as they run for too long, although they are runned automatically on Travis when merged into master. Run browser tests locally in any browser:
 
     $ npm run test-browser-local