travis: For ghc-8.6, switch to use 8.6.5

Author: erikd

.travis.yml

@@ -9,7 +9,7 @@ env:
  - GHCVER=8.0.2
  - GHCVER=8.2.2
  - GHCVER=8.4.4
- - GHCVER=8.6.2
+ - GHCVER=8.6.5
 
 before_install:
  - sudo add-apt-repository -y ppa:hvr/ghc