Don't try to run tests on GHC 7.4

There's no point in trying to run tests on GHC 7.4, as tasty-rerun
requires base-4.6.

Signed-off-by: Maximilian Marx <mmarx@wh2.tu-dresden.de>
Relax dependency version pins to allow building on GHC 7.8

Signed-off-by: Maximilian Marx <mmarx@wh2.tu-dresden.de>