Exclude the /libs/ directory from linting, refs T167

Test Plan:
- Modify jquery.js
- Run `arc lint`
- No lint errors reported

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: lkamau, eyedol

Maniphest Tasks: T167

Differential Revision: https://phabricator.ushahidi.com/D25