Fixed Cabal version typo.

Author: svenpanne

.travis.yml

@@ -25,7 +25,7 @@ matrix:
     - env: CABALVER=1.24 GHCVER=8.2.2
       compiler: ": #GHC 8.2.2"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.2.2], sources: [hvr-ghc]}}
-    - env: CABALVER=1.24 GHCVER=8.4.1
+    - env: CABALVER=2.0 GHCVER=8.4.1
       compiler: ": #GHC 8.4.1"
       addons: {apt: {packages: [cabal-install-2.0,ghc-8.4.1], sources: [hvr-ghc]}}
     - env: CABALVER=head GHCVER=head