Update Hugo test versions

Author: EmielH

.travis.yml

@@ -4,7 +4,8 @@ env:
   - HUGO_VERSION="0.52"
   - HUGO_VERSION="0.53"
   - HUGO_VERSION="0.54.0"
-  - HUGO_VERSION="0.55.1"
+  - HUGO_VERSION="0.55.6"
+  - HUGO_VERSION="0.56.3"
 
 install:
   - wget https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_${HUGO_VERSION}_Linux-64bit.tar.gz