Fix three.js Vector3 typing

threejs/three.d.ts

@@ -3674,7 +3674,7 @@ declare module THREE {
         /**
          * Adds v to this vector.
          */
-        add(a: Object): Vector3;
+        add(a: Vector): Vector3;
         addScalar(s: number): Vector3;
 
         /**
@@ -5759,4 +5759,4 @@ declare module THREE {
 
 declare module 'three' {
     export=THREE;
-}
+}