fix(typescript): invalid JSDoc annotation (#148)

Also, fixed the TS config to detect this error in future.

Author: JLarky

typings/react-popper.d.ts

@@ -21,7 +21,7 @@ interface PopperProps {
   children: (props: ({
     ref: (ref: HTMLElement | null) => void,
     style: React.CSSProperties,
-    placement: ?PopperJS.Placement,
+    placement?: PopperJS.Placement,
     outOfBoundaries: boolean | null,
     scheduleUpdate: () => void,
     arrowProps: {

typings/tests/tsconfig.json

@@ -5,7 +5,6 @@
     "jsx": "react",
     "noEmit": true,
     "strict": true,
-    "skipLibCheck": true,
     "moduleResolution": "node",
     "typeRoots": ["./node_modules/@types"],
     "types": ["react"]