replace another couple of deferreds

src/client.js

@@ -1872,9 +1872,8 @@ MatrixClient.prototype.joinRoom = function(roomIdOrAlias, opts, callback) {
 
     const reqOpts = {qsStringifyOptions: {arrayFormat: 'repeat'}};
 
-    const defer = Promise.defer();
-
     const self = this;
+    const prom = new Promise((resolve, reject) => {
         sign_promise.then(function(signed_invite_object) {
             const data = {};
             if (signed_invite_object) {
@@ -1894,11 +1893,12 @@ MatrixClient.prototype.joinRoom = function(roomIdOrAlias, opts, callback) {
             }
             return Promise.resolve(room);
         }).done(function(room) {
-        _resolve(callback, defer, room);
+            _resolve(callback, resolve, room);
         }, function(err) {
-        _reject(callback, defer, err);
+            _reject(callback, reject, err);
         });
-    return defer.promise;
+    });
+    return prom;
 };
 
 /**
@@ -3214,12 +3214,8 @@ MatrixClient.prototype.scrollback = function(room, limit, callback) {
     // reduce the required number of events appropriately
     limit = limit - numAdded;
 
-    const defer = Promise.defer();
-    info = {
-        promise: defer.promise,
-        errorTs: null,
-    };
     const self = this;
+    const prom = new Promise((resolve, reject) => {
         // wait for a time before doing this request
         // (which may be 0 in order not to special case the code paths)
         sleep(timeToWaitMs).then(function() {
@@ -3241,15 +3237,22 @@ MatrixClient.prototype.scrollback = function(room, limit, callback) {
             }
             self.store.storeEvents(room, matrixEvents, res.end, true);
             self._ongoingScrollbacks[room.roomId] = null;
-        _resolve(callback, defer, room);
+            _resolve(callback, resolve, room);
         }, function(err) {
             self._ongoingScrollbacks[room.roomId] = {
                 errorTs: Date.now(),
             };
-        _reject(callback, defer, err);
+            _reject(callback, reject, err);
         });
+    });
+
+    info = {
+        promise: prom,
+        errorTs: null,
+    };
+
     this._ongoingScrollbacks[room.roomId] = info;
-    return defer.promise;
+    return prom;
 };
 
 /**
@@ -4862,18 +4865,18 @@ function checkTurnServers(client) {
     });
 }
 
-function _reject(callback, defer, err) {
+function _reject(callback, reject, err) {
     if (callback) {
         callback(err);
     }
-    defer.reject(err);
+    reject(err);
 }
 
-function _resolve(callback, defer, res) {
+function _resolve(callback, resolve, res) {
     if (callback) {
         callback(null, res);
     }
-    defer.resolve(res);
+    resolve(res);
 }
 
 function _PojoToMatrixEventMapper(client) {