Revert "travis: follow SF.net download URL change"

This reverts commit 75a78fcf80a87daa94531d3433dd7a690e73b512.

Because it causes the following warning:

     W: Failed to fetch http://sourceforge.net/projects/cutter/files/ubuntu/dists/precise/main/binary-amd64/Packages
     Got a single header line over 360 chars