fix(middleware): do not pass next to wrapFn

passing next to wrapFn causes nested error stack

src/Group.js

@@ -66,13 +66,12 @@ class Group {
    * Wraps the test/hook as a middleware fn
    *
    * @param   {Object}   fn
-   * @param   {Promise} next
    * @return  {Promise}
    *
    * @private
    */
-  _wrapFn (fn, next) {
-    return new Promise((resolve, reject) => fn.run().then(next).then(resolve).catch((error) => {
+  _wrapFn (fn) {
+    return new Promise((resolve, reject) => fn.run().then(resolve).catch((error) => {
       reject({title: fn._title, error: error})
     }))
   }

src/Middleware.js

@@ -51,7 +51,8 @@ class Middleware {
 
     const fn = this._stack[index]
     return new Promise((resolve, reject) => {
-      this._fnWrapper(fn, next => this._dispatch(index + 1))
+      this._fnWrapper(fn)
+      .then(() => this._dispatch(index + 1))
       .then(resolve)
       .catch((error) => this._internalRejection.bind(this)(index, resolve, reject, error))
     })

src/Runner.js

@@ -145,13 +145,12 @@ class Runner {
    * Wraps the group.run as a middleware function.
    *
    * @param  {Function}
-   * @param  {Function}
    * @return {Promise}
    *
    * @private
    */
-  _wrapFn (fn, next) {
-    return new Promise((resolve, reject) => fn.run().then(next).then(resolve).catch(reject))
+  _wrapFn (fn) {
+    return new Promise((resolve, reject) => fn.run().then(resolve).catch(reject))
   }
 
   /**