[Development] Closing the 5.5 branch

[Frederik Gladhorn]

Hi all,

due to the strain that we put on the VMs that are supposed to get the releases 
out and do the testing, we decided to "shut down" the CI for Qt 5.5 branches.
This means literally that we shut down the VMs but we'll keep them around in 
case we ever need to make a 5.5 release.
The goal is to free capacity to let us finally get 5.6 and just after 5.7 out 
of the door.

Cheers,
Frederik