Map props.key to state.intl.locale (#6)

See proposed solution to switching locales with react-redux formatjs/formatjs#234 (comment)
I tweaked mapStateToProps to set props.key to state.intl.locale so that a re-render occurs when locale is hot swapped.

Author: macca16

src/components/Provider.js

@@ -7,6 +7,7 @@ const mapStateToProps = (state) => {
   const {intl} = state
   return {
     ...intl,
+    key: intl.locale,
   }
 }