Disable Haddock for GHC >= 8.4.1, it uses far too much memory.

* GHC 8.2.2/Haddock 2.18.1: Haddock uses 3.1GB maximum RSS
* GHC 8.4.1/Haddock 2.19.0: Haddock uses 4.4GB maximum RSS

The latter is too much for Travis-CI.

Fun fact: Given enough RAM, the actual use/syste/wall clock time remains
basically constant across the versions.

.travis.yml

@@ -42,9 +42,10 @@ matrix:
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-8.2.2], sources: [hvr-ghc]}}
     - compiler: "ghc-8.4.1"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
+      env: HADDOCK=false
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-8.4.1], sources: [hvr-ghc]}}
     - compiler: "ghc-head"
-      env: GHCHEAD=true
+      env: GHCHEAD=true HADDOCK=false
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-head], sources: [hvr-ghc]}}
 
   allow_failures: