Update underscore.js to 1.13.6

vendor/underscore/modules/_optimizeCb.js

@@ -0,0 +1,21 @@
+// Internal function that returns an efficient (for current engines) version
+// of the passed-in callback, to be repeatedly applied in other Underscore
+// functions.
+export default function optimizeCb(func, context, argCount) {
+  if (context === void 0) return func;
+  switch (argCount == null ? 3 : argCount) {
+    case 1: return function(value) {
+      return func.call(context, value);
+    };
+    // The 2-argument case is omitted because we’re not using it.
+    case 3: return function(value, index, collection) {
+      return func.call(context, value, index, collection);
+    };
+    case 4: return function(accumulator, value, index, collection) {
+      return func.call(context, accumulator, value, index, collection);
+    };
+  }
+  return function() {
+    return func.apply(context, arguments);
+  };
+}