Add support for code standards in root of repository for PHP_CodeSniffer 3.x

[frenck]

Problem/Motivation

PHP_CodeSniffer requires the coding standards to be in a directory, this behaviour changed in 3.x.

"The installed_paths config setting can now point directly to a standard. Previously, it had to always point to the directory in which the standard lives"

See the release notes:
https://github.com/squizlabs/PHP_CodeSniffer/releases/tag/3.0.0a1

This plugin does not support this yet.

Expected behaviour

Allowing a repository with a coding standard in the root of is project to work, when PHP_CodeSniffer 3.x or newer is used.

Actual behaviour

The coding standard won't be installed by this plugin.

Proposed changes

Change the coding standards search depth based on the PHP_CodeSniffer version used.

[frenck]

Added in #30, closing issue.