Updates and moar security

• Use roave/security-advisories package to prevent installing packages with known vulnerabilities
• Add WP_CACHE_KEY_SALT constant to allow object cache isolation
• Update rarst/laps package to next major version (2.0)
• Update all packages, sync composer.lock
• Micro-optimisation: replace dirname( __FILE__ ) with __DIR__
• Remove unnecessary parentheses from language constructs (require, require_once)
• Update README