.travis.yml: Added package docbook-xml
.travis.yml: Added package docbook-xml

a2x (asciidoc) depends on the Docbook XML DTD contained in the
docbook-xml package.

authmillenon/libcoap,gebart/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap