[Tests] Added travis_retry to .travis.yml

| Q             | A
| ------------- | ---
| Bug fix?      | -
| New feature?  | -
| BC breaks?    | -
| Deprecations? | -
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

As stated on their [blog](http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/), this is the recommended way.

Example of a failed build because of it:
https://travis-ci.org/symfony/symfony/jobs/17664775