Require librsvg2 for SVG conversion on MediaWiki SaaS role

Summary:
Generally installed as an ImageMagick dependency, but to be safe,
the mediawiki_software_dependencies state entry now requires it.

Ref T845.

Test Plan: no op

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T845

Differential Revision: https://devcentral.nasqueron.org/D1492