Made PHP version in composer.json more flexible

Changed PHP version number in composer.json from fixed value 5.3.18
to range >=5.3.18,<6.0.0
This allows composer, grunt etc to run with the latest versions of
XAMPP which include PHP version 5.4.x
Limited compatibility to 5.x.x in case PHP 6.0 changes something
dramatically.

This was the only real problem I encountered following the screencast. Specifying a range of PHP version numbers avoids the problem. Everything seems to work OK with PHP version 5.4.7. Latest versions are 5.4.15 and 5.3.25. Also 5.5.0RC2 is out so 5.5.0 may not be too far away...