feat(package.json): update to react@^15.0.0
feat(package.json): update to react@^15.0.0

* Permit React 15-rc to be included as a peer dependency.
* Update to 15.0.0 instead of the release candidate version.
* Closes #243
RishabhJain96/react-google-maps,joshcarr/react-google-maps,jossi87/react-google-maps,RishabhJain96/react-google-maps,jossi87/react-google-maps,tomchentw/react-google-maps