Bump the default lts to 13.3.

Also test ghc-8.6.3 in Travis CI instead of 8.6.1.

.travis.yml

@@ -11,8 +11,8 @@ matrix:
     addons: {apt: {packages: [cabal-install-2.0, ghc-8.2.2], sources: [hvr-ghc]}}
   - env: CABALVER=2.2 GHCVER=8.4.3
     addons: {apt: {packages: [cabal-install-2.2, ghc-8.4.3], sources: [hvr-ghc]}}
-  - env: CABALVER=2.4 GHCVER=8.6.1
-    addons: {apt: {packages: [cabal-install-2.4, ghc-8.6.1], sources: [hvr-ghc]}}
+  - env: CABALVER=2.4 GHCVER=8.6.3
+    addons: {apt: {packages: [cabal-install-2.4, ghc-8.6.3], sources: [hvr-ghc]}}
 
 install:
  - export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH

stack.yaml

-resolver: lts-9.14
+resolver: lts-13.6
 
 packages:
 - .