Update TinkerPop related links

[FlorianHockmann]

The TinkerPop links point to version 3.2.4 of the TinkerPop docs. When we can't update them automatically then I would suggest to simply point to the current version of the docs.
Apart from that, the texts sound a bit odd. Gremlin graph server should simply be Gremlin Server and we could probably replace the links to Gremlin applications with a link for the Gremlin Console as the server and the console are the applications most users will be interested in. A link to the GLVs might also make sense but I guess that's already covered by #31.

The three OLAP links should probably also be reduced to just Spark as Hadoop MapReduce wasn't ever supported in TinkerPop 3 to my knowledge and support for Giraph will be dropped with TinkerPop 3.4.0 (see TINKERPOP-1930) so we shouldn't promote it anymore. Otherwise new users might choose Giraph over Spark and will then be disappointed when they can't move forward to TinkerPop 3.4.0 and therefore newer JanusGraph versions.