Don't run travis builds on PRs

Currently travis builds require secret env variables which are not accessible when running PR builds. So, it's better to turn off travis for PRs.

As per

http://docs.travis-ci.com/user/pull-requests/#Security-Restrictions-when-testing-Pull-Requests here's a snippet that does this:

```
script:
  - '[ "${TRAVIS_PULL_REQUEST}" = "false" ] && bundle exec rake tests:integration || false'
```
