prophecy: [symfony 3.1.x compatibility] Use phpdocumentor/reflection-docblock 3.0

Hello, I encountered with a conflict that will eventually prevent users to update to Symfony 3.1 composer output:

phpunit/phpunit 5.* requires phpspec/prophecy, 
phpspec/prophecy requires phpdocumentor/reflection-docblock ~2.0
​phpdocumentor/reflection-docblock ~2.0  conflicts with symfony 3.1*​

phpdocumentor/reflection-docblock already has 3.0 stable version https://github.com/phpDocumentor/ReflectionDocBlock/issues/70

Please see conflict section https://packagist.org/packages/symfony/symfony#3.1.x-dev