Include PHPCS path in `$PATH`. Install WP standards for PHPCS.

Before:

```bash
/srv/www/phpcs/scripts/phpcs --standard=CodeSniffer/Standards/WordPress/WordPress/ruleset.xml ./htdocs/wp-load.php
```

After:
```bash
phpcs --standard=WordPress ./htdocs/wp-load.php
```
@simonwheatley this needs to be relative to the root of the PHPCS repo, as otherwise PHPCS will not be able to find the standards when invoked outside of Vagrant. So it should be:

```bash
phpcs --config-set installed_paths CodeSniffer/Standards/WordPress/
```

See https://github.com/squizlabs/PHP_CodeSniffer/pull/217