[TravisCI] Retry the installation of cutter.

On TravisCI, the installtion of cutter sometimes failure as below.
So we retry it if it fails.

$ curl https://raw.github.com/clear-code/cutter/master/data/travis/setup.sh | sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   560  100   560    0     0   9138      0 --:--:-- --:--:-- --:--:-- 12173
deb http://downloads.sourceforge.net/project/cutter/ubuntu/ precise main
deb-src http://downloads.sourceforge.net/project/cutter/ubuntu/ precise main
W: Failed to fetch http://downloads.sourceforge.net/project/cutter/ubuntu/dists/precise/Release.gpg  Connection failed
E: Some index files failed to download. They have been ignored, or old ones used instead.