Update ruby versions supported by Rails 6

Author: wpolicarpo

.travis.yml

@@ -15,10 +15,6 @@ script:
   - docker-compose run ci
 matrix:
   include:
-    - name: 2.3.8
-      env: TARGET_VERSION=2.3.8
-    - name: 2.4.6
-      env: TARGET_VERSION=2.4.6
     - name: 2.5.5
       env: TARGET_VERSION=2.5.5
     - name: 2.6.3