Enzyme: Deprecate use with React 0.13

[blainekasten]

Curious on the other maintainers thoughts on deprecating use of this library with React 0.13 moving forward. Of course the previous versions are still usable. But it seems that the conditional requirement of test-utils causes plenty of confusion.

Deprecating 0.13 would allow us to clean up some code, and reduce confusion.

Thoughts?

cc @Aweary, @nfcampos, @ljharb, @lelandrichardson

[ljharb]

I prefer to support the latest major, plus the previous 2. When v16 comes out, I'd be happy to immediately ship a breaking change that drops support for 0.13.

That said, "deprecation" is not breaking, and I think it's reasonably to support N, N-1, and N-2, but to have N - 2 be officially deprecated.

[nfcampos]

@blainekasten yep removing support for 0.13 would remove most (all?) of the conditional requires, which would be great to reduce confusion. would it actually remove all of them?

[ljharb]

Now that enzyme v3 is out, and the 0.13 stuff is in its own adapter, there's not really a reason to ever drop support for it.