added version constraints

src/compiler/gf.cabal

@@ -58,7 +58,7 @@ flag network-uri
 executable gf
   main-is: gf-main.hs
   default-language:    Haskell2010
-  build-depends: pgf2,
+  build-depends: pgf2 >= 4.0.0,
                  base >= 4.6 && <5,
                  array,
                  containers,

src/server/pgf-service.cabal

@@ -23,7 +23,7 @@ Library
                    filepath,
                    containers,
                    process,
-                   pgf2 >= 2,
+                   pgf2 >= 4.0.0,
                    http-slim,
                    mtl,
                    json >= 0.3.3,