Fix jquery.d.ts break with TypeScript 'develop' branch

The beta compiler is smarter about detecting inconsistencies:
jquery.d.ts(192,11): error TS2189: Interface 'JQueryEventObject' cannot simultaneously extend types 'BaseJQueryEventObject' and 'JQueryInputEventObject':
Types of property 'metaKey' of types 'BaseJQueryEventObject' and 'JQueryInputEventObject' are not identical.

Documentation seems to indicate 'boolean' is correct: http://api.jquery.com/event.metaKey/
Merge pull request #1128 from derekcicerone/patch-18

Fix jquery.d.ts break with TypeScript 'develop' branch