Use community searchkick as it now includes wordnet

@amoose submitted a patch (ankane/searchkick#250) to include support
for wordnet and we were previously using his patched fork. It has now
been included in the master branch of searchkick so I am switching back
to the community version.
Merge branch 'feature/use_community_searchkick' into dev

This fixes #216. I'm not sure exactly what the issue was but I'm not
going to look into it further unless the problem returns.