Fix apt-get crash in Travis CI

Author: Michaël Dupont

.travis.yml

@@ -36,6 +36,7 @@ env:
 
 before_install:
     ## Packages
+    - sudo rm -f /etc/apt/sources.list.d/mongodb.list  # Makes apt crash on Precise, and we don't need MongoDB
     - sudo apt-get update -qq
     - sudo apt-get install -y graphviz