Update Travis to only run tests using the latest 2.3.1 (for now)

Our Travis build queue has been slow and we are trying to get a number
of new samples tested and merged.

After the upcoming samples are merged, we will add all current, stable
Ruby versions to our Travis matrix:

 - 2.3.1
 - 2.2.5
 - 2.1.10

Author: remi Taylor

.travis.yml

@@ -15,8 +15,7 @@
 sudo: false
 language: ruby
 rvm:
-- 2.2.3
-- 2.0.0
+- 2.3.1
 
 branches:
   only: