Fixes #1153

Previously the `Phaser.Keyboard` module's `lastKey` property was of type
`string` in the TypeScript definition file, but it is actually a
`Phaser.Key` object, so `build/phaser.d.ts` has been updated to reflect
that.

### P.S.

Considering that you want pull requests against the `dev` branch, it'd be convenient to make that the default branch for this repo on GitHub.