Add back airship for GHC 8
Add back airship for GHC 8

Removed unnecessary ImpredicativeTypes and now compiles under GHC 8.
int-index/stackage,GregorySchwartz/stackage,fpco/stackage,brendanhay/stackage,jaspervdj/stackage,jonschoning/stackage,wereHamster/stackage,codedmart/stackage,iand675/stackage,mrkkrp/stackage,athanclark/stackage,christiaanb/stackage,alanz/stackage,nomeata/stackage,Daniel-Diaz/stackage,ygale/stackage,k0ral/stackage,phaazon/stackage,peti/stackage,simonmichael/stackage,clinty/stackage,lwm/stackage,bartavelle/stackage,soenkehahn/stackage,asr/stackage,stackbuilders/stackage,phadej/stackage,fimad/stackage,Shimuuar/stackage,creichert/stackage,judah/stackage,garetxe/stackage,michalkonecny/stackage,ethercrow/stackage,l29ah/stackage,mstksg/stackage,DanielG/stackage,spencerjanssen/stackage,mgajda/stackage,massysett/stackage