DATASOLR-92 - switch to data-commons 1.6.0.BUILD-SNAPSHOT
DATASOLR-69 - test enum conversion from/to solr document

Simple enums are handled automatically using MappingSolrConverter.
Registering additional converters takes a little bit of configuration as
those converters have to be added to CustomConversions, which have to
be passed on to MappingSolrConverte. This MappingSolrConverter has
to be set as converter for SolrTemplate. DATASOLR-89 will cover the
configuration issue.