Set `onKey` for Many-to-Many Through associations

Seems to have slipped through https://github.com/balderdashy/waterline-schema/commit/af0976145e7a8deddc3a7204940e5665066debbb and fixes https://github.com/balderdashy/waterline/pull/507
Merge pull request #11 from mphasize/master

Set `onKey` for Many-to-Many Through associations