Update Ruby version matrix on CI

https://github.com/postmodern/ruby-versions/blob/3fc3154ae59668e518d271db58c095332478795c/ruby/stable.txt

Author: Juanito Fatas

.travis.yml

@@ -3,13 +3,11 @@ sudo: false
 cache: bundler
 before_install: gem update bundler --no-document
 rvm:
-  - 1.9.3
-  - 2.0.0
-  - 2.1.10
-  - 2.2.9
-  - 2.3.6
-  - 2.4.3
-  - 2.5.0
+  - 2.2
+  - 2.3
+  - 2.4
+  - 2.5
+  - 2.6
   - ruby-head
   - jruby
 matrix: