Use ppa for newer doxygen

.travis.yml

@@ -26,6 +26,7 @@ before_install:
   - sudo add-apt-repository ppa:ubuntugis/ppa -y
   - sudo add-apt-repository ppa:ubuntugis/ubuntugis-unstable -y # For postgresql-9.1-postgis-2.1
   - sudo add-apt-repository ppa:smspillaz/cmake-3.0.2 -y
+  - sudo add-apt-repository ppa:kedazo/doxygen-updates-precise -y # For doxygen 1.8.8
   - sudo rm -f /etc/apt/sources.list.d/pgdg-source.list # postgis from pgdg requires different gdal package than the grass package
   - sudo apt-get update -qq
   - sudo apt-get remove postgresql-9.1-postgis-2.1 # Remove postgis from pgdg, will install postgis from ubuntugis-unstable instead