[BIS2027] add `Subject` model

and make it exposable via API at endpoint `/v1/subjects`
[BIS2027] add subject feature to scheme (admin side)
[BIS2027] add *Food and Drink* sector
[BIS2027] permit `subject_ids` on api endpoint
fix travis build

update source for `bitzesty/import_csv` and don't attempt to remove `.rspec`
[BIS2027] regenerate api documentation
update gems
Merge pull request #34 from bitzesty/BIS2027

[BIS2027] add `subject` filter