Problem: documentation build not tested on Travis

Solution: install asciidoc and xmlto in the libsodium build run, since
it already refreshes APT to download other packages, so the delay is
smaller

.travis.yml

@@ -41,6 +41,8 @@ matrix:
           key_url: 'http://download.opensuse.org/repositories/home:/zeromq:/git-stable/xUbuntu_14.04/Release.key'
         packages:
         - libsodium-dev
+        - asciidoc
+        - xmlto
   - env: BUILD_TYPE=default CURVE=libsodium
     os: osx