Use a regular expression to parse out React.version

This fixes a silent failure of the test suite after whatever grunt step follows `version-check`. The failure appears to be due to the presence of the call `require('./build/modules/React')` in the `version-check` step, but I am not sure why it fails.

This is also nice because it no longer requires evaluating the entire React dependency tree just to check the version.

cc @zpao @spicyj