Use stack-7.8.yaml for Cabal builds on GHC 7.8
Use stack-7.8.yaml for Cabal builds on GHC 7.8

Also, override compiler version in resolver to prevent Stack failures.

juhp/stack,AndrewRademacher/stack,juhp/stack,MichielDerhaeg/stack,anton-dessiatov/stack,deech/stack,deech/stack,Fuuzetsu/stack,AndrewRademacher/stack,martin-kolinek/stack,AndreasPK/stack,mrkkrp/stack,Blaisorblade/stack,Blaisorblade/stack,martin-kolinek/stack,AndreasPK/stack,anton-dessiatov/stack,mrkkrp/stack,MichielDerhaeg/stack,Fuuzetsu/stack