Update Ruby versions on Travis CI

* Remove 2.0 & 2.1
* Add 2.4

Author: tricknotes

.travis.yml

@@ -3,9 +3,8 @@ language: ruby
 bundler_args: "--binstubs --standalone --without documentation --path ../bundle"
 script: "bundle exec rake test"
 rvm:
-  - 2.0.0
-  - 2.1.5
   - 2.2.0
   - 2.3.3
+  - 2.4.0
 notifications:
   email: false