Make stopwords actually strings.

Summary:
Certain tokens, like "on", parse as True in YAML without the quotes.
This leads to unexpected results.

Test Plan:
 - ran it against dev domain
 `python provision-domain.py khan-academy-dev-2 domain-info.yaml`

Reviewers: zach

Reviewed By: zach

Subscribers: johnsullivan, jessie

Differential Revision: http://phabricator.khanacademy.org/D12445