Re-consider PHPCS `installed_paths` argument

As identified in #96, our coding standards <= v0.5.0 cannot handle missing paths gracefully. In v0.6.0 we get improved handling for missing sniffs but we projects are still defaulted to 0.5.0 and we have projects set to run even older versions of the standards.

We need a good way of using newer coding standard sets that we add, such as the VIPCS or the PHPCompatibility standards.

My initial inclination is to parse the [standardPath](https://github.com/humanmade/linter-bot/blob/master/src/linters/phpcs/index.js#L40) to determine what version is being used and only load in newer standards for 0.6.0 and above. The downside is that this is hacky and we'll have to account for `latest` being on 0.5.0 for the next month.

Any thoughts @rmccue @kevinlangleyjr?