corrected signature of mountComponent to current version

this is cherry picked from fix-for-react-changes (see pull request https://github.com/facebook/react-art/pull/20) against master (instead of 0.10-stable) for easier pull request

i do not know/understand react and art enough to come up with a reasonable test.  I just made it because observed runtime type errors are fixed with this. I got these runtime errors by using react-art from om so i'm not sure i can provide a minimal testcase. The fix is purely based on symmetry with other mountComponent function signatures in ReactART.js.