Use https:// for media-embed tests

I overlooked this when reviewing #6418 since it is excluded on travis
because of the @nophantomjs tag (so it didn't fail). But since we block
http:// embeds now, we need to use https:// for the tests.

features/desktop/media-embed.feature

@@ -13,10 +13,10 @@ Feature: oembed
     And I sign in as "alice@alice.alice"
 
   Scenario: Post a video link
-    When I click the publisher and post "[title](http://example.com/file.ogv)"
+    When I click the publisher and post "[title](https://example.com/file.ogv)"
     Then I should see a HTML5 video player
 
   Scenario: Post an audio link
-    When I click the publisher and post "[title](http://example.com/file.ogg)"
+    When I click the publisher and post "[title](https://example.com/file.ogg)"
     Then I should see a HTML5 audio player