INTEGRATION_PORT documented but not implemented

The test description in [Rakefile](https://github.com/ruby-ldap/ruby-net-ldap/blob/master/Rakefile) says "INTEGRATION_HOST and INTEGRATION_PORT are also supported".

[test/test_helper.rb](https://github.com/ruby-ldap/ruby-net-ldap/blob/master/test/test_helper.rb) uses `INTEGRATION_HOST`, but port is hardcoded to 389 for integration tests; there is no other reference to `INTEGRATION_PORT`.
