Switch build.kde.org to Qt stable branch

[Nicolás Alvarez]

As you may know, Qt is working to release 5.2. The dev branch has been
merged to the stable branch, which means the stable branch is now 5.2,
and dev is now 5.3.

build.kde.org is compiling Qt from source, but it's using the dev
branch. As far as I know, the plan is to release KF5 depending on Qt
5.2. I propose switching build.kde.org to the 'stable' Qt branch to
ensure we don't accidentally depend on 5.3 features.

-- 
Nicolás