Update dashif:laurl to dashif:Laurl per latest specification

[Gregory McGarry]

See discussion here: Dash-Industry-Forum/dash.js#4459

mp4-dash.py creates a dashif:laurl element with the URL to the clearkey license server. The name of this element has been updated in the spec to dashif:Laurl. There are a few other changes for the clear-key case.

Here is at patch:

diff --git a/Source/Python/utils/mp4-dash.py b/Source/Python/utils/mp4-dash.py
index 70ac4a6..5d42860 100755
--- a/Source/Python/utils/mp4-dash.py
+++ b/Source/Python/utils/mp4-dash.py
@@ -312,7 +312,7 @@ def AddContentProtection(options, container, tracks, all_tracks):
             ck_license.text = options.clearkey_license_uri
 
             # Second entry with the dashif namespace
-            ck_license = xml.SubElement(cp, '{'+DASHIF_NAMESPACE+'}laurl')
+            ck_license = xml.SubElement(cp, '{'+DASHIF_NAMESPACE+'}Laurl')
             ck_license.text = options.clearkey_license_uri
 
     # Marlin