Fixed ARIA role attribute for button and slider

src/js/button.js

@@ -43,7 +43,7 @@ vjs.Button.prototype.createEl = function(type, props){
   props = vjs.obj.merge({
     className: this.buildCSSClass(),
     innerHTML: '<div class="vjs-control-content"><span class="vjs-control-text">' + (this.buttonText || 'Need Text') + '</span></div>',
-    role: 'button',
+    'role': 'button',
     'aria-live': 'polite', // let the screen reader user know that the text of the button may change
     tabIndex: 0
   }, props);

src/js/slider.js

@@ -40,7 +40,7 @@ vjs.Slider.prototype.createEl = function(type, props) {
   // Add the slider element class to all sub classes
   props.className = props.className + ' vjs-slider';
   props = vjs.obj.merge({
-    role: 'slider',
+    'role': 'slider',
     'aria-valuenow': 0,
     'aria-valuemin': 0,
     'aria-valuemax': 100,