Update CS setup

[jrfnl]

The current code style and PHP compatibility setup is pretty out-of-date, inconsistent and convoluted:

• The Travis script seems to try use your theme to unit test PHPCompatibility.
  Honestly, you don't have to as PHPCompatibility does extensive unit testing itself.
  Just run it once on a high PHP version with a properly set testVersion and you're done - or in case you want to test separately for compatibility with PHP 5.6 and above and compatibility with PHP 5.2/5.3-5.5, run it twice, but running it the way it's done now.... is really not very effective.
  See: https://github.com/PHPCompatibility/PHPCompatibility#sniffing-your-code-for-compatibility-with-specific-php-versions
• As this is a WordPress theme, you probably should use PHPCompatibilityWP instead of PHPCompatibility.
  See: https://github.com/PHPCompatibility/PHPCompatibilityWP
• The Composer file is pointing to the abandoned wimg/php-compatibility repo.
  The package was moved to phpcompatibility/php-compatibility a year ago....
• squizlabs/php_codesniffer is not a direct dependency for your theme, but a dependency of PHPCompatibility(WP). Don't require it, but let the package which depends on it manage the supported versions.
• The DealerDirect PHPCS Composer plugin has released a new version and should be updated to use ^0.5.0.
  Composer treats minors < 1.0 as majors, so this needs to be done explicitly.
  See: https://github.com/Dealerdirect/phpcodesniffer-composer-installer/releases
• You seem to include a codesniffer.ruleset.xml file, but don't require-dev the needed dependency (WPCS) for it.
• The WPCS version which the ruleset seems to be based on is very much out of date and a number of sniffs have been renamed since. This ruleset will not work anymore.
• The ruleset has a non-standard name which means that PHPCS will not pick up on it automatically.
  Please consider renaming the file to phpcs.xml.dist.
• The code is not actually checked against the ruleset via Travis anyway, so what is going on here ?
• Please consider using the WP ThemeReview ruleset: https://github.com/WPTRT/WPThemeReview

If this is truly intended as a starter-theme, I would strongly suggest leading by example and setting these things up properly and correctly.