Update version to 4.15.0

Signed-off-by: Erik de Bruin <erik@ixsoftware.nl>
Fix Maven URL to avoid 'SSL' error due to redirect

Signed-off-by: Erik de Bruin <erik@ixsoftware.nl>
FLEX-25045 getString now returns null as expected if supplied a valid locale but not a valid resourceName
FLEX-25045 added mustella test for issue
Add ability to use legacy mobile theme.
Fixes https://issues.apache.org/jira/browse/FLEX-34685
Merge commit '1884960242f835879f5ca6504659072a2305f120' into release4.14.0