fix: proptype warnings (jquense#1084)

Author: prizef

src/EventCell.js

@@ -16,7 +16,7 @@ let propTypes = {
   accessors: PropTypes.object.isRequired,
   components: PropTypes.object.isRequired,
   getters: PropTypes.object.isRequired,
-  localizer: PropTypes.object.isRequired,
+  localizer: PropTypes.object,
 
   onSelect: PropTypes.func,
   onDoubleClick: PropTypes.func,

src/Popup.js

@@ -26,7 +26,7 @@ const propTypes = {
   localizer: PropTypes.object.isRequired,
   onSelect: PropTypes.func,
   onDoubleClick: PropTypes.func,
-  slotStart: PropTypes.number,
+  slotStart: PropTypes.instanceOf(Date),
   slotEnd: PropTypes.number,
 }