.travis.yml: Fix CABALVER for GHC 8.10

Cabal 3.2 is the earliest version with support for GHC 8.10, despite the GHC release having taken place first.

Author: Deewiant

.travis.yml

@@ -4,7 +4,7 @@ env:
  - CABALVER=2.2 GHCVER=8.4.4 # Debian 10
  - CABALVER=2.4 GHCVER=8.6.5
  - CABALVER=3.0 GHCVER=8.8.3
- - CABALVER=3.0 GHCVER=8.10.1
+ - CABALVER=3.2 GHCVER=8.10.1
  - CABALVER=head GHCVER=head
 
 language: generic