Remove libxmljs from released dependencies

BREAKING:

Library libxmljs is not installed any more when installing osprey-method-handler.
This was the initially intended behaviour/output. So far libxmljs was added as a package dependency by mistake because of an incorrect Travis configuration.

Projects that use XML validation but not specify libxmljs as an explicit dependency will break. To make such a project work with new osprey-method-handler versions, please make sure to install libxmljs yourself.