Update minimum requirement

[sdieunidou]

Lexik mailer is not compatible anymore with version of Symfony < 3 (invalid syntax on services.xml since you have adding the "default" section).

The minimum requirement of symfony dependency is currently invalid since LexikMailer 2.0.9.

  [Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]                                        
  Unable to parse file "/symfony/vendor/lexik/mailer-bundle/DependencyInjection/../Resources/config/services.xml".  
                                                                                                                    

                                                                                                                                                                                                              
  [InvalidArgumentException]                                                                                                                                                                                  
  [ERROR 1871] Element '{http://symfony.com/schema/dic/services}defaults': This element is not expected. Expected is ( {http://symfony.com/schema/dic/services}service ). (in /symfony/ - line 41, column 0)