Establish a convention for forcing jsx rebuilds

Pull request #526 updated the behavior of `vendor/constants.js` without changing any source files or the `bin/jsx-internal` script, so files that should have been rebuilt (like `utils/__tests__/ImmutableObject-test.js`) were not automatically rebuilt (unless you knew to do `rm -rf .module-cache` manually).

This commit allows us to bump a version number (`commonerCacheBuster`) when we know the transform toolchain has been altered in a way that will not be visible to commoner/jsx.

With this convention, if we reset to an older revision (e.g. during a `git bisect`) and the appropriate cached module files are still in the `.module-cache/`, they can be used without rebuilding. That's why I prefer this approach to just deleting the `.module-cache/`.

cc @subtleGradient @zpao

Closes #104.
Closes #496.
Closes #530.