chore(ci): Add Ruby 2.6, drop Ruby 2.3

Author: DirtyF

.travis.yml

@@ -4,11 +4,11 @@ cache: bundler
 matrix:
   include:
     - # GitHub Pages
-      rvm: 2.5.1
+      rvm: 2.5.3
       env: GH_PAGES=true
 rvm:
-  - 2.5
-  - 2.3
+  - 2.6
+  - 2.4
 before_install:
 - gem update --system
 before_script: bundle update