Suppress hsc2hs-related warning on GHC HEAD

Untabify and delete trailing whitespace

This makes haskeline `-Wtabs`-clean

Relax upper bound to allow upcoming `transformers-0.5`

GHC 8.0 will ship with the not yet released `transformers-0.5.0.0`
release.

Generalise constraints for InputT instances

Canonicalise AMP instances

This flips the AMP instance definitions into canonical form and makes
the code a bit more future proof.

I'm not sure why the code was written the way it was, but it seems we
can do better by letting GHC directly reuse `ReaderT`'s instances as
well as relaxing the constraints from

    instance Monad m       => Functor     (InputT m)
    instance Monad m       => Applicative (InputT m)
    instance Monad m       => Monad       (InputT m)

to

    instance Functor     m => Functor     (InputT m)
    instance Applicative m => Applicative (InputT m)
    instance Monad       m => Monad       (InputT m)

This looks the same on GHC 7.8 (pre-AMP) as well as GHC 7.10 (post-AMP)

Fix Linux to Windows cross-compile

The #include of <Shlobj.h> doesn't work when cross-compiling from
Linux to Windows because the Linux file system is case sensitive and
on Linux the file is <shlobj.h>. This change should not affect Windows
builds because the Windows file system is case insensitive.

GHC 8 is gonna come with base-4.9

Updated to latest version of multi-ghc-travis.

The build on 7.2.* was failing due to the constraint "Cabal >= $CABALVER"
which tried to build a too-recent version of Cabal.  Fixed by changing the
constraint to "Cabal == $CABALVER.*".

Allow new filepath-1.4 for GHC 7.10.1RC3

See haskell/filepath@d039d5ae

Conflicts:
	haskeline.cabal

This simplies the installation with GHC < 7.6.

Fix TODO: messageState param not needed anymore

Use explicit forall syntax to avoid warning

This is currently just a warning in GHC HEAD, but ought to be fixed
nevertheless:

    Variable ‘m’ is implicitly quantified due to a context
    Use explicit forall syntax instead.
    This will become an error in GHC 7.12.