Update test cases for GHC bug #8945, Haddock #188

The order of signature groups has been corrected upstream. Here we add a
test case and update some existing test-cases to reflect this change. We
remove grouped signature in test cases that we can (Minimal,
BugDeprecated &c) so that the test is as self-contained as possible.
Enforce strict GHC version in cabal file

This stops people with 7.6.3 trying to install 2.15.x which clearly
won't work. Unfortunately we shipped 2.14.x without realising this.
Initialise some new PlatformConstants fields