Fixed acceptance test for start time / end time slider range.

It turned out to be that when you run the tests in
cms/djangoapps/contentstore/features/video.feature
all at once, then test # 11 starts with transcripts already turned on. When you
run just the test # 11, then transcripts are not available (from previous tests).
That's why to make width consistent, we have to make sure that transcripts are
hidden.

cms/djangoapps/contentstore/features/video.feature

@@ -81,6 +81,7 @@ Feature: CMS.Video Component
   # 11
   Scenario: When start end end times are specified, a range on slider is shown
     Given I have created a Video component
+    And Make sure captions are closed
     And I edit the component
     And I set value "12" to the field "Start Time"
     And I set value "24" to the field "End Time"