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/.

Closes #104.
Closes #496.
Closes #530.