Bumped the versioning of Symfony components

[aurimasniekis]

No description provided.

[hacfi]

@aurimasniekis ~3.4 isn’t really necessary if ~3.0 is already there

[brianfreytag]

This will not pass tests as of current. All of the dynamically-created services are done without a privacy set, so where they were public by default up through 3.4, they are now private in 4.0. There are tests that run to ensure they can be pulled from the container.

I've submitted a PR (#34) that both makes the bundle pass tests and updates the composer.json definitions to include SF ~4.0 and also add a PHP7.2 test to the travis configuration.