travis.yml: add ghc 8,6.5 and 8.8.1 to the list of tested compilers.

Author: peti

.travis.yml

@@ -38,6 +38,10 @@ before_cache:
 
 matrix:
   include:
+    - compiler: "ghc-8.8.1"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-3.0,ghc-8.8.1], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.6.5"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.4,ghc-8.6.5], sources: [hvr-ghc]}}
     - compiler: "ghc-8.4.4"
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.4,ghc-8.4.4], sources: [hvr-ghc]}}
     - compiler: "ghc-8.2.2"
@@ -54,6 +58,7 @@ matrix:
   allow_failures:
     - compiler: "ghc-7.6.3"
     - compiler: "ghc-7.8.4"
+    - compiler: "ghc-8.8.1"
 
 before_install:
   - HC=/opt/ghc/bin/${CC}