Add support for checking minimum PHP version in XML update stream

This introduces a <php_minimum> tag which should indicate the minimum supported PHP version of an extension
Optimize version_compare check, thanks @dongilbert
Add message if an update is available but unsupported
Restructure PHP version check
Merge remote-tracking branch 'joomla/staging' into phpUpdateLib
Merge branch 'staging' into phpUpdateLib
Data object is a stdClass instance
Merge pull request #2802 from mbabker/phpUpdateLib

Add support for checking minimum PHP version in XML update stream