Update travis.ci to support docbook/fop doc generation

Author: vitcpp

Diff for: .travis.yml

@@ -16,7 +16,7 @@ dist: focal
 before_install:
   # extra apt.pg.o.sh options added in version 204, travis currently has 199 (2019-11-27)
   - sudo apt-get -qq update
-  - sudo apt-get -y install postgresql-common libhealpix-cxx-dev docbook-dsssl docbook-xml openjade
+  - sudo apt-get -y install postgresql-common libhealpix-cxx-dev docbook-xml docbook-xsl libxml2-utils xsltproc fop
 
 install:
   - sudo /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh -p -v $PG_SUPPORTED_VERSIONS -i