PHP Code Sniffer 1.5.1

Updated to version 1.5.1.

Version 1.5.1 added a new installed_paths config value to allow for the
setting of directories that contain standards.

The formula adds #{etc}/php-code-sniffer/Standards as such as
configuration. This will let other formulas install standards and link
them there - then they will survive an upgrade of php-code-sniffer.