match up the cabal versions in travis better

Author: cartazio

.travis.yml

@@ -30,7 +30,7 @@ matrix:
   include:
     - compiler: "ghc-8.6.2"
       env: TESTTARGETS="vector:vector-tests-O0" # https://ghc.haskell.org/trac/ghc/ticket/13535
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.6.2], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.4,ghc-8.6.2], sources: [hvr-ghc]}}
     - compiler: "ghc-8.4.3"
       env: TESTTARGETS="vector:vector-tests-O0" # https://ghc.haskell.org/trac/ghc/ticket/13535
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.4.3], sources: [hvr-ghc]}}