only test on latest patch version of ruby

Author: bolandrm

.travis.yml

@@ -3,7 +3,6 @@ language: ruby
 bundler_args: "--binstubs --standalone --without documentation --path ../bundle"
 script: "bundle exec rake test"
 rvm:
-  - 2.3.3
   - 2.3.7
   - 2.4.4
   - 2.5.1