Only set related_name on ProxyGenericRelation when absolutely necessary

To work around Django bugs in <1.6. Fixes #204 for Django >= 1.6.

(cherry picked from commit 18582c77f36d9af1ee93d79afa35941221ea32ab)

This merges #264 and fixes #204.

Reviewed-By: Antoine Catton <acatton@fusionbox.com>