Fix play/pause DOMException (ringcentral#131)

* Fix play/pause DOMException (ringcentral#1)
* `playPromise` audio instance fix

Author: mangatinanda

Diff for: src/ringcentral-web-phone.js

@@ -77,14 +77,19 @@
                 this._audio[url].src = url;
                 this._audio[url].loop = true;
                 this._audio[url].volume = volume;
-                this._audio[url].play();
+                this._audio[url].playPromise = this._audio[url].play();
             }
         } else {
             if (val) {
                 this._audio[url].currentTime = 0;
-                this._audio[url].play();
+                this._audio[url].playPromise = this._audio[url].play();
             } else {
-                this._audio[url].pause();
+                var audio = this._audio[url];
+                if (audio.playPromise !== undefined) {
+                    audio.playPromise.then(function() {
+                        audio.pause();
+                    });
+                }
             }
         }