Allow usage with Symfony 4

[PetrHeinz]

It seems that this package is in fact compatible with Symfony 4 already, so it should be enoughto change its requirements in composer.json. I've created this little PR to add the support. It will be incompatible with Symfony 5 though due to changes in FormTypeExtensionInterface.

I've added some jobs to the Travis build to support my claims. Builds on Symfony >=4.2 fail due to deprecations - it succeeds on 4.0 and Symfony is pretty good with keeping backward compatibility. Jobs with hhvm and Docker fail even on current master (see build of bd2cd20 in my fork or restart the build yourself).

Are you still able to merge and release a new version (3.1.0, I'd suppose), or is this project no longer maintained? Can I help? Thanks :slightly_smiling_face:

[ghost]

Author of this bundle is not active anymore. Different package, but same author. https://symfony.com/blog/finding-a-new-home-for-ivoryckeditorbundle