Restore build with ghc-7.10.3

gf.cabal

@@ -81,7 +81,9 @@ library
     mtl >= 2.2.1 && < 2.3,
     pretty >= 1.1.3 && < 1.2,
     random >= 1.1 && < 1.3,
-    utf8-string >= 1.0.1.1 && < 1.1
+    utf8-string >= 1.0.1.1 && < 1.1,
+    -- We need this in order for ghc-7.10 to build
+    transformers-compat >= 0.6.3 && < 0.7
 
   if impl(ghc<8.0)
     build-depends:

src/compiler/GF/Interactive.hs

@@ -38,6 +38,8 @@ import GF.Server(server)
 #endif
 
 import GF.Command.Messages(welcome)
+-- Needed to make it compile on GHC < 8
+import Control.Monad.Trans.Instances ()
 
 -- | Run the GF Shell in quiet mode (@gf -run@).
 mainRunGFI :: Options -> [FilePath] -> IO ()