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

GoogleCloudPlatform/ruby-docs-samples,remi/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,remi/ruby-docs-samples,remi/ruby-docs-samples