Marshmallow 4.x support

[mockodin]

Marshmallow 4 has hit GA is there plan for adding support in flask-rebar?

I took a quick look I see issues with:

• when sorted is called in marshmallow_to_swagger
• marshmallow-objects

Happy to contribute coding time if desired

[mockodin]

Added #326

[illusional]

Thanks @mockodin for raising this, and creating the PR! This has been released in 3.4.0 😄