Be specific on video progress slider handle

Using `.ui-slider-handle` was too broad.

lms/static/coffee/spec/modules/video/video_progress_slider_spec.coffee

@@ -18,7 +18,7 @@ describe 'VideoProgressSlider', ->
           stop: @slider.onStop
 
       it 'build the seek handle', ->
-        expect(@slider.handle).toBe '.ui-slider-handle'
+        expect(@slider.handle).toBe '.slider .ui-slider-handle'
         expect($.fn.qtip).toHaveBeenCalledWith
           content: "0:00"
           position:

lms/static/coffee/src/modules/video/video_progress_slider.coffee

@@ -17,7 +17,7 @@ class @VideoProgressSlider
     @buildHandle()
 
   buildHandle: ->
-    @handle = @$('.ui-slider-handle')
+    @handle = @$('.slider .ui-slider-handle')
     @handle.qtip
       content: "#{Time.format(@slider.slider('value'))}"
       position: