.travis.yml: upgrade ruby to v2.6.2

https://www.ruby-lang.org/en/downloads/releases/
Fixes https://github.com/Kickball/awesome-selfhosted/issues/1653

.travis.yml

@@ -4,7 +4,7 @@ node_js:
   - "node"
 
 before_install:
-  - rvm install 2.2
+  - rvm install 2.6.2
   - gem install awesome_bot
   - gem install danger