* Add React.isValidElementType()
Per the conversation on #12453, there are a number of third-party
libraries (particularly those that generate higher-order components)
that are performing suboptimal validation of element types.
This commit exposes a function that can perform the desired check
without depending upon React internals.
* Move isValidElementType to shared/
* Support ForwardRef type of work in TestRenderer and ShallowRenderer.
* Release script now updates inter-package dependencies too (e.g. react-test-renderer depends on react-is).
