Set PHP_CodeSniffer installed_path config for folder restructuring

Depends on:
 * https://github.com/squizlabs/PHP_CodeSniffer/pull/217 (merged)
 * https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/pull/194 (unmerged)

With this change, there will be additional standards available, including:

 * `WordPress` (current superset of all sniffs)
 * `WordPress-Core` (sniffs that seek to implement, and go no farther than, the official coding standards)
 * `WordPress-VIP` (Core sniffs plus sniffs specifically implemented to check against the [VIP coding requirements](http://vip.wordpress.com/documentation/code-review-what-we-look-for/))
 * `WordPress-Extra` (Core sniffs plus any extras that are best practices but could be controversial)