[fixed] remove es2015 from v1 branch
[fixed] remove es2015 from v1 branch

closes #375

springload/react-modal,seanich/react-modal,springload/react-modal,seanich/react-modal,springload/react-modal,diasbruno/react-modal,bluSCALE4/react-modal,reactjs/react-modal,reactjs/react-modal,diasbruno/react-modal,bluSCALE4/react-modal,rackt/react-modal,diasbruno/react-modal,bluSCALE4/react-modal,seanich/react-modal,reactjs/react-modal,rackt/react-modal