Fine-tune lower bound on lib:Cabal

GHC 8.4.1-rc1 (GHC 8.4.0.20180224) unfortunately still shipped with a
devel snapshot of Cabal-2.1.0.0; but GHC 8.4.1 final will ship w/
lib:Cabal-2.2.0.0

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -1039,7 +1039,7 @@ planPackages verbosity comp platform solver SolverSettings{..}
     -- respective major Cabal version bundled with the respective GHC
     -- release).
     --
-    -- GHC 8.4   needs  Cabal >= 2.1  (GHC 8.4.1-rc1 has Cabal-2.1)
+    -- GHC 8.4   needs  Cabal >= 2.2
     -- GHC 8.2   needs  Cabal >= 2.0
     -- GHC 8.0   needs  Cabal >= 1.24
     -- GHC 7.10  needs  Cabal >= 1.22
@@ -1050,6 +1050,10 @@ planPackages verbosity comp platform solver SolverSettings{..}
     -- TODO: long-term, this compatibility matrix should be
     --       stored as a field inside 'Distribution.Compiler.Compiler'
     setupMinCabalVersionConstraint
+      | isGHC, compVer >= mkVersion [8,4,1] = mkVersion [2,2]
+        -- GHC 8.4.1-rc1 (GHC 8.4.0.20180224) still shipped with an
+        -- devel snapshot of Cabal-2.1.0.0; the rule below can be
+        -- dropped at some point
       | isGHC, compVer >= mkVersion [8,4]  = mkVersion [2,1]
       | isGHC, compVer >= mkVersion [8,2]  = mkVersion [2,0]
       | isGHC, compVer >= mkVersion [8,0]  = mkVersion [1,24]