Change default value for $wgMathLaTeXMLUrl

new LaTeXML instance.
Sponsored by https://www.xsede.org/

Cherry-Pick from Id92ef141c7921a82528b3ba4384e026322499312

Change-Id: Idd1de9da348d473acc6cea8d2d1a855f1f6b07fa
Merge "Change default value for $wgMathLaTeXMLUrl"