Update Ruby to 2.6

Author: Nicolai Parlog

.github/workflows/actions.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Set up Ruby
         uses: actions/setup-ruby@v1
         with:
-          ruby-version: 2.5
+          ruby-version: 2.6
       - name: Set up Java
         uses: actions/setup-java@v1
         with: