try to fix Travis build

Author: jmckenna

.travis.yml

@@ -29,7 +29,7 @@ language: c
 before_install:
   - sudo mv /etc/apt/sources.list.d/pgdg* /tmp
   - sudo apt-get purge -y libgdal* libgeos* libspatialite*
-  - sudo add-apt-repository -y ppa:ubuntugis/ubuntugis-unstable
+  - sudo add-apt-repository -y ppa:ubuntugis/ubuntugis-stable
   - sudo apt-get update
   - sudo apt-get install cmake libspatialite-dev libfcgi-dev libproj-dev libgeos-dev libgdal-dev libtiff-dev libgeotiff-dev apache2-dev libpcre3-dev libsqlite3-dev libdb-dev
 # For testing