Add a linter rule for curly brace array indexes

Summary: In PHP, both $x['key'] and $x{'key'} can be used to access an array (see http://stackoverflow.com/questions/8092248/php-curly-braces-in-array-notation), but the former should be preferred.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14603