Add extension.json support

[mwjames]

This PR is made in reference to: #

This PR addresses or contains:

• Adds support for extension.json hereby requires wfLoadExtension( "SemanticCite" ); to be added to LocalSettings.php in order to enable the extension
• Change PHP min. requirement to PHP 5.6+

This PR includes:

• [ ] Tests (unit/integration)
• [x] CI build passed

[kghbln]

@mwjames Should I do a 1.4.0 release before this gets merged? Not many changes but still worth doing.