Behat has arrived!

* Added Behat, Mink and Behat Symfony extension to our Composer file
* Added a first simple "top nav menu" feature
Added a "RepositoryEmptier" Trait, used in existing Commands

This not only factorizes the code of our existing Commands, but this new Trait should also be very handy for functional testing too :-)
RepositoryEmptier Trait : let's use Entities classes aliases
WIP on Behat tests setup

* Added a "app_test.php" Symfony Front Controller
* Added a "testing" Nginx server in the Dockerfile, mapped to "app_test.php"
* Added a "the following N books" Gherkin definition to fill the DB
* Added some default values for some Books Entities properties
* Added a specific, hard-coded DB name in "config_test.yml"
Added a "homepage" Behat feature

+ added some ids and classes to some of our homepage HTML elements
Added a "newsletter subscription" Behat feature
Added a "CountableEntityRepository" Trait, used on Newsletter Subscriptions
First Travis CI integration test
"SymfonyRequirements.php" has been updated by the last Symfony upd8
Travis CI setup bugfix
Travis CI setup bugfix, bis
Travis CI setup bugfix, ter
Travis CI setup bugfix, episode IV
Travis CI setup bugfix, episode V - including some "print last response" debug additions to our features
Travis CI setup bugfix, episode VI
Travis CI setup bugfix, episode VII - the git pushes strike back

Took some Travis install directives from @Sylius :-)
Travis CI setup bugfix, episode VIII

Our Behat Mink driver still receives blank pages, this is very frustrating :-/
Let's try with another port...
Travis CI setup bugfix, episode IX
Travis CI setup bugfix, episode X

Added some debugging "tail" calls in .travis.yml ; maybe I will finally understand why this Travis config don't work?
Travis CI setup bugfix, episode XI

Replacing Apache server with Symfony's built-in server...
(tailored for "test" env)