Fix for 500 when using the Topic filter

If you were to use the Topic filter with a search term, it was 500ing from
an ambiguous column name in contact_search.rb, specifically in the include
of contact_groups, which wasn't needed for the search to function.
Merge pull request #92 from alphagov/fixing-500-on-search-topic-filter-use

Fix for 500 when using the Topic filter