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

.travis.yml

@@ -15,7 +15,7 @@ matrix:
 services: mongodb
 
 before_script:
-    - sudo apt-get install parallel
+    - travis_retry sudo apt-get install parallel
     - sh -c 'if [ "$TRAVIS_PHP_VERSION" != "hhvm" ]; then echo "" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini; fi;'
     - sh -c 'if [ "$TRAVIS_PHP_VERSION" != "hhvm" ]; then echo "extension = mongo.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini; fi;'
     - sh -c 'if [ "$TRAVIS_PHP_VERSION" != "hhvm" ] && [ $(php -r "echo PHP_MINOR_VERSION;") -le 4 ]; then echo "extension = apc.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini; fi;'