restrict ruby testing to version 2.3.3

Signed-off-by: Patrick Münch <[email protected]>

Author: atomic111

.travis.yml

@@ -2,9 +2,7 @@
 language: ruby
 cache: bundler
 rvm:
-  - 2.0
-  - 2.2
-  - 2.3.1
+  - 2.3.3
 
 bundler_args: --without integration
 script: bundle exec rake