bump ruby versions

Author: DirtyF

.travis.yml

@@ -1,10 +1,11 @@
 language: ruby
 script : script/cibuild
+cache: bundler
 sudo: false
 
 rvm:
-  - 2.3.1
-  - 2.2.5
+  - 2.3.3
+  - 2.2.6
   - 2.1.9
 env:
   - ""
@@ -13,5 +14,5 @@ env:
 matrix:
   include:
     - # GitHub Pages
-      rvm: 2.1.1
+      rvm: 2.3.3
       env: GH_PAGES=true