Add SaxonHE to dependencies list

Used by Boost.Beast in doc reference generation.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

.travis.yml

@@ -33,6 +33,7 @@ addons:
       - texlive
       - sshpass
       - ghostscript
+      - libsaxonhe-java
   ssh_known_hosts: frs.sourceforge.net
 
 env: