Bump validation timeout to 180s (bug 1068339)

https://bugzilla.mozilla.org/show_bug.cgi?id=1068339

The 110s timeout is inherited from AMO, but we are dealing with some packaged apps that can be quite bigger than addons, and some do trigger the timeout. Bumping it a little should alleviate the issue, it's not a proper fix but it's better than letting those apps fail.