Add LDAP (with SASL) support to PHP.
LDAP support for PHP.

Ensure libldap-dev is installed and linked in /usr/lib/.
Related to #332.
Disable LDAP on PHP 5.2.17 and 5.3.3.

There are compilation errors with these old versions.
Related to #332.
Fix zmq's PHP extension preinstallation.

Related to travis-ci/travis-ci#2323.