Forge Release: 2.4.2

• README: Making changes due to Puppet Approved review - MODULES-7737
• PHP Agent: Added support for Debian 9 natively using PHP7 - #21
• Metadata: Adding upper bound to Puppet version support and signalling Puppet 6 support
• Metadata: Now supporting puppetlabs/apt version 6.x and puppetlabs/stdlib version 5.x - #22
• PDK: Now compatible with PDK 1.9.0