Drop older dependencies

[Jean85]

This PR tries to bump up the versions required by this package. I'm not sure if this will require a major version bump, but we can decide this later; in fact, no functionality has been changed, only ::class is used everywhere was possible so the PHP version constraint should take care of everything.

In summary, this PR implements this changes:

• raise requirement to PHP 5.6
• bump version of PHPUnit to ^5.4|^6
• bump CS fixer to latest version
• simplify the Travis CI matrix
• eliminates Symfony requirement from the CI (it wasn't needed!)
• add Scrutinizer
• add test coverage collection