Bump jstransform dependency to point at 4.0.1 or higher

As noted in https://github.com/facebook/react/issues/1454, jstransform 4.0.0 accidentally bumped it's dependency on `source-map` down to 0.1.8 (from 0.1.31).

jstransform@4.0.1 fixes this -- so bump react's dependency to point at that nothing before that version