indexing, role-based access, and bulk download fixes

• upgrade dor-services to the latest version (currently 5.8.2) to fix formatting of workflow step completion/error dates in solr docs. this was preventing indexing of most things, since solr rejected the dates as being in an invalid format. see issue #758.
• fix construction of queries for listing APOs and collections to which a user has access. this was preventing role based access from working properly. see issue #759.
• add a .xml extension to the descMD files in MODS bulk download. see issue #756.
• update nokogiri to 1.6.8 (address vulnerability that failed bundle audit).

in the case of the first two bugs, argo and dor-services had been doing the wrong thing for a very long time, but solr had previously been tolerant of our malformed requests (an upgrade to solr 6.1 brought stricter behavior from solr).