Travis: added extras-precise source package

Author: Clement Champetier

.travis.yml

@@ -76,7 +76,7 @@ after_failure:
 
 addons:
   apt:
-    sources: ['boost-latest', 'kalakris-cmake', 'kubuntu-backports', 'pythonxy-devel']
+    sources: ['boost-latest', 'kalakris-cmake', 'kubuntu-backports', 'extras-precise', 'pythonxy-devel']
     packages:
     - cmake
     - swig