Avoid access to private properties

Requires https://github.com/SemanticMediaWiki/SemanticMediaWiki/commit/16b8f6c4887b692aed089fa42575036c62602a81
Merge pull request #29 from SemanticMediaWiki/valuedesc

Avoid access to private properties
Update INSTALL.md
Merge branch 'master' of ssh://wmfgerrit/mediawiki/extensions/SemanticMaps