Don't process pusher events immediately when running in background

When user has Travis CI's web page open in the background we should not
process pusher events immediately, because she will not see the changes
on a page anyway. During peak hours we can get several messages per
second, which requires some CPU work almost all the time. By using
visibility API we can detect if the tab with Travis CI is in the
background and if that's the case we will process pusher messages in
batches every 5 minutes. This is much better for the CPU, because it
means one bigger spike every few minutes and additionally some of the
updates don't have to be done (for example if we get several job:started
events, Ember's run loop will process only last one when it comes to
rendering 'current' build).