Remove core-js import
Remove core-js import

`Symbol` is now part of ECMAScript and core-js is not included by default in babel 7 builds. 

By removing the explicit import, developers can use their own transpile process - if `Symbol` does not exist in their version of javascript, they'll get an error about that.
glortho/react-keydown,jedverity/react-keydown,jedverity/react-keydown,glortho/react-keydown