Add a requirement in tests installation instructions

PHPUnit requires Yaml package from pear.symfony.com channel.
Fix tests execution

See #2442
Try a different method, the previous one wasn't working in Travis
Merge pull request #2462 from vlakoff/develop-4

Fix tests execution
Merge pull request #2460 from vlakoff/develop-3

Add a requirement in tests installation instructions
Merge pull request #2 from rogeriopradoj/upstream-develop

Update fork with official