Allow "$regex" inside "$not" queries on Mongo 4

[arieltorti]

Fix for #81

Mongo 4.* now allows "$regex" inside "$not" queries:

 {"$not": {"$regex": "(?i)Hello World"}}

This patch modifies the behavior so that it's allowed in Mongo 4.* but not in Mongo 3.*

[davidlatwe]

Hey @arieltorti , The code looks really good and thanks for the fast contribution. 👍🏼

I have nothing to add, let me know if you are happy to merge and I'll merge it.

[arieltorti]

Awesome, let's merge it then. Could we do a release including these changes?

[davidlatwe]

Yeah, will make a 2.5.1 release later once merged :)