Add a custom setup to cabal-install.

This custom setup ensures that if we are bootstrapping, we will
use the most recent version of Cabal.  (Without setup, cabal-install
will force usage of an old version of Cabal.)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-install/cabal-install.cabal

@@ -356,3 +356,9 @@ test-suite integration-tests
 
   ghc-options: -Wall
   default-language: Haskell2010
+
+custom-setup
+  setup-depends: Cabal >= 1.23,
+                 base,
+                 process   >= 1.1.0.2  && < 1.5,
+                 filepath   >= 1.3      && < 1.5