This ought to fix the build matrix. I believe the issue is GHC trac
5129, where evaluate is being miscompiled: in cabal-testsuite's
Test.Cabal.Server, in the function readUntilEnd, the 'read' function
is protected by an 'if' to ensure that it doesn't result in the
dreaded 'no parse'. However, some sophisticated putStrLn
debugging has revealed that it is being called in *both* branches, not
just the safe one; this is the worst sort of heisenbug because it
vanished when I put a putStrLn before the evaluate!

Since the evaluate function is fixed in 8.4.2 and bumping to 8.4.2
has a patch for trac 5129, this is as fixed as it can get.