Fix Animated.loop binding. (#459)

Author: cknitt

reason-react-native/src/apis/Animated.md

@@ -252,7 +252,7 @@ type loopConfig;
 external loop: Animation.t => Animation.t = "";
 
 [@bs.module "react-native"] [@bs.scope "Animated"]
-external loopWithConfig: (Animation.t, loopConfig) => Animation.t = "";
+external loopWithConfig: (Animation.t, loopConfig) => Animation.t = "loop";
 
 type animatedEvent;
 

reason-react-native/src/apis/Animated.re

@@ -245,7 +245,7 @@ type loopConfig;
 external loop: Animation.t => Animation.t = "";
 
 [@bs.module "react-native"] [@bs.scope "Animated"]
-external loopWithConfig: (Animation.t, loopConfig) => Animation.t = "";
+external loopWithConfig: (Animation.t, loopConfig) => Animation.t = "loop";
 
 type animatedEvent;