Dropping support for NHC98 and old GHC from containers

The containers package currently includes CPP and Cabal directives intended to support NHC98 and versions of GHC before 7.0. However, our TravisCI script tests only GHC versions 7.0 and above. It is therefore likely that the compatibility code is bit-rotted to some extent. Wren Romano and I are strongly inclined to immediately remove all code intended to support NHC98 or pre-7.0 GHC. If anyone uses any such implementations with the current version of containers, please speak now or forever hold your peace. Thanks, David Feuer