Put explicit lower bound on version of base

It does not build with ghc-6.4.2, missing Functor instance for Either.

cabal-install/cabal-install.cabal

@@ -67,7 +67,8 @@ Executable cabal
         Distribution.Client.Win32SelfUpgrade
         Paths_cabal_install
 
-    build-depends: Cabal >= 1.6 && < 1.7,
+    build-depends: base >= 2 && < 4,
+                   Cabal >= 1.6 && < 1.7,
                    filepath >= 1.0,
                    network >= 1 && < 3,
                    HTTP >= 3000 && < 3002,
@@ -76,7 +77,7 @@ Executable cabal
     if flag(old-base)
       build-depends: base < 3
     else
-      build-depends: base       >= 3   && < 4,
+      build-depends: base       >= 3,
                      process    >= 1   && < 1.1,
                      directory  >= 1   && < 1.1,
                      pretty     >= 1   && < 1.1,