Added travis_retry to .travis.yml

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
minor #10142 [Tests] Added travis_retry to .travis.yml (lavoiesl)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #10142).

Discussion
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[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

Commits
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702432a Added travis_retry to .travis.yml