MINOR: Change field type to better index
MINOR: Change field type to better index

stephenmcm/silverstripe-solr,nyeholt/silverstripe-solr,stephenmcm/silverstripe-solr,nyeholt/silverstripe-solr,nyeholt/silverstripe-solr,nyeholt/silverstripe-solr,stephenmcm/silverstripe-solr,stephenmcm/silverstripe-solr