Change requirements to latest version instead of specific version

[mayk93]

After installing drf-mongo-filter, my Django, restframework, pymongo and mongoengine versions were all downgraded.

Please use == instead of ~= in the requirements file or issue a warning before downgrade.

[qwiglydee]

The "~=" indeed allows to match any of minor version.

But the project is quite old and probably incompatible with new libraries anyway.