trying to update cmake to 2.8.12 in travis via a ppa repo

Whitelist of repos:
https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json

Whitelist of packages:
https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-precise

.travis.yml

@@ -20,5 +20,13 @@ matrix:
     - os: osx
       env: BUILD=Doc
 
+addons:
+  apt:
+    sources:
+    - kubuntu-backports                   # cmake 2.8.12
+#    - george-edison55-precise-backports   # cmake 3.2.3
+    packages:
+    - cmake
+
 script:
   - support/travis-build.py