Remove peer dependency on react-test-renderer

[SimenB]

I don't use React in my project, but I get a noisy peer dep warning from both npm and yarn when installing this module.

As it's optional, do you think you could drop it as a peer dep?

[thymikee]

I'm supportive to that change. I'll tag it as a good first issue

Note to implementer:
The change requires us to put react-test-renderer call in a try/catch and throw some useful error when user tries to use it. We also need a test.