Declare proper spec version in `base.cabal`
Declare proper spec version in `base.cabal`

This is a follow-up to ba2ae2c8729d5aef2aeb7fb32d6c0ea2a465ea25
which started relying on a new Cabal feature requiring the
the cabal spec version declaration to be updated accordingly.

sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc