restrict ruby testing to version 2.3.3

Signed-off-by: Patrick Münch <patrick.muench1111@gmail.com>

Author: atomic111

.travis.yml

@@ -3,9 +3,7 @@ language: ruby
 cache: bundler
 
 rvm:
-  - 2.0
-  - 2.2
-  - 2.3.1
+  - 2.3.3
 
 bundler_args: --without integration
 script: bundle exec rake