Update Symfony Support

[franmomu]

These are the changes:

• Remove Silex ~1.3 as is not supported since long time ago (I've just realised that there is a Silex 2.4 which has been deprecated for a year, but this one supports symfony ^4.0 packages, so I'm not sure if it should be introduced again)
  - Use phpunit 7.5 and for that I had to use also mockery > 1.0 and there was an error with nette/util that was fixed on 2.4
• And for Symfony I bumped to the maintained versions

Closes #234

[florianeckerstorfer]

Awesome, can you fix the conflict and make the tests pass?

[franmomu]

I bumped phpunit to 6.5 and mockery to 1.2 so the php constraint with this is ^7.0. Taking a look at #246, don’t know which versions are going to be dropped, I could make a PR just dropping 5.x or wait for that PR.

[florianeckerstorfer]

@franmomu If you have the time to create a PR to drop PHP 5 support, that would be awesome.

[franmomu]

I have no idea why scrutinizer fails