rails: fix API calls which include non-person relationships
rails: fix API calls which include non-person relationships

This fixes API calls like:

http://localhost:3000/api/v1/organisations/194/meetings?include=government-offices
http://localhost:3000/api/v1/organisations/194/meetings?include=organisations

HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence