Support un-braced element prop in JSX transform

Like `<LeftRight left=<a /> right=<b /> />`. facebook/esprima#9 makes this possible.

This should be merged with an esprima-fb version bump. However, facebook/esprima@8ca1599a1b3599fb3666f1094d9a0c226cf5ef5b necessitates a change to this transform to look at `object.openingElement` in most cases so I'll wait for an updated version to be synced out before this can be merged.