Remove PHPCS 2.X support

[tomjn]

WPCS 1.0 is deprecating support for PHPCS 2, which means we won't be able to support PHPCS 2 anyway. We should do the same, and remove the polyfills added to make things work with v2 of PHPCS

[GaryJones]

Items seen in passing:

• Remove ^2.9.0 || constraint in composer.json.
• PHPCS 3.x requires PHP 5.4+, so the "php": ">=5.3" requirement in composer.json can be updated to something more recent.
• Remove the autoloading of the PHPCSCompatibility.php file.
• Remove the PHPCSCompatibility.php file, which loaded an aliases file from WPCS.
• Update implementations of \PHP_CodeSniffer_Sniff to be PHP_CodeSniffer\Sniffs\Sniff.
• Update single extends of \PHP_CodeSniffer_Standards_AbstractScopeSniff to be PHP_CodeSniffer\Sniffs\AbstractScopeSniff.
• Update any other PHPCS 2.x classes, per the alias file. Tests/ already seem to be using PHP_CodeSniffer\Tests\Standards\AbstractSniffUnitTest.
• Remove PHP 5.3 / PHPCS 2.9 references from .travis.yml.

[GaryJones]

Remove the autoloading of the PHPCSCompatibility.php file.
Remove the PHPCSCompatibility.php file, which loaded an aliases file from WPCS.

While we still rely on WPCS classes directly, some aliases need to be maintained, either by referencing the WPCS aliases file, or including an aliases file in VIPCS.