MediaWiki's rv* parameters are deprecated

[jimsafley]

See https://lists.wikimedia.org/pipermail/mediawiki-api-announce/2017-June/000134.html

While rvdifftotext, rvdiffto, rvexpandtemplates,and rvgeneratexml are present in the MediaWiki service class, we only use rvdiffto (to get the difference between two revisions). Remove them all from the $_actions list. Instead of revids/rvdiffto in getRevisionDiff(), use action=compare. (Note that the compare action was not added until 1.18 and the fromid/toid properties were not added until 1.20.)