Few more return statements in hopes of squashing warnings

Not sure if this will have any effect since finally or tap do not have any effect on what happens next in the change so there's no issue with returning undefined.

lib/transaction.js

@@ -185,8 +185,9 @@ Transaction.prototype.commit = function() {
     .finally(function() {
       self.finished = 'commit';
       if (!self.parent) {
-        self.cleanup();
+        return self.cleanup();
       }
+      return null;
     });
 };
 
@@ -212,8 +213,9 @@ Transaction.prototype.rollback = function() {
     .finally(function() {
       self.finished = 'rollback';
       if (!self.parent) {
-        self.cleanup();
+        return self.cleanup();
       }
+      return null;
     });
 };
 
@@ -241,6 +243,7 @@ Transaction.prototype.prepareEnvironment = function() {
     if (self.sequelize.constructor.cls) {
       self.sequelize.constructor.cls.set('transaction', self);
     }
+    return null;
   });
 };