Disable Travis snapshots deploy for pull requests

As @brasmusson pointed out in #536, Travis should not try to deploy snapshots for pull requests. This pull request should enable deploy when they are merged only.

Some concerns:
* Anyone could re-enable it by commenting out those lines and sending a pull request
* Each artifact is deployed as soon as the tests pass, but I think they should be deployed only if the entire build succeeds (after_success phase)