Update GHC versions, include 8.8.4

.travis.yml

@@ -12,12 +12,10 @@ cache:
 
 jobs:
   include:
+  - ghc: 8.8.4
+    env: SNAPSHOT="lts-16.20"
   - ghc: 8.6.5
     env: SNAPSHOT="lts-14.22"
-  - ghc: 8.6.4
-    env: SNAPSHOT="lts-13.19"
-  - ghc: 8.6.3
-    env: SNAPSHOT="lts-13.11"
   - ghc: 8.4.4
     env: SNAPSHOT="lts-12.26"
 

stack.yaml

@@ -15,7 +15,7 @@
 # resolver:
 #  name: custom-snapshot
 #  location: "./custom-snapshot.yaml"
-resolver: lts-14.21
+resolver: lts-16.20
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.

validationt.cabal

@@ -18,6 +18,7 @@ extra-source-files:  ChangeLog.md
 cabal-version:       >=1.10
 tested-with:         GHC == 8.4.4
                    , GHC == 8.6.5
+                   , GHC == 8.8.4
 
 source-repository head
   type:     git