Change licence of PHP wrapper back to LGPL-2.1+

This is a revival of a8bef56

Still reachable authors gave their approval here:
http://sourceforge.net/p/ming/mailman/message/32503963/

The change was needed to prevent Debian from moving the php wrapper
in the "nonfree" repository, and to fix an incompatibility in binary
packages.

Closes #42