feat(GoogleMap): add warning for undefined google object
feat(GoogleMap): add warning for undefined google object

* Closes #142

ksavenkov/react-google-maps,RishabhJain96/react-google-maps,jossi87/react-google-maps,RishabhJain96/react-google-maps,tomchentw/react-google-maps,jossi87/react-google-maps,joshcarr/react-google-maps