Symfony 3 compatibility

[pascaldevink]

I've shamelessly copied the proposed changes by @kierate (from michelsalib/BCCResqueBundle#127) and @mrardon (from michelsalib/BCCResqueBundle#136) to get the symfony 3 compatibility complete, which fixed #3.
I hope they don't mind me using their work to push this bundle forward.

It should be compatible with both Symfony 2.8 and 3.0|3.1, since the location of the bootstrap.php.cache file is now configurable.

[mpclarkson]

Thanks so much @pascaldevink

[mpclarkson]

@pascaldevink let me know if you'd like to be a collaborator on this project - I don't have the bandwidth to maintain it myself.

[pascaldevink]

@mpclarkson As my company is heavily relying on resque and the resque bundle, I'm trying to actively participate in any progress that it might have.
That being said, I think most of this bundle is actually pretty stable and it doesn't require a bunch of new features (other than nice-to-haves people have come up with).

So yes, I would like to be a collaborator on this project, but I'd propose even going a step further: how about introducing a special github organisation for this project? That would:

1. make it a lot easier to ensure the future of the bundle;
2. make it easier to transfer the bundle to whoever wants to maintain it in the future;
3. make the namespace agnostic to the maintainer

I've opened a new issue to propose this, so we can discuss it in there.