Fix jruby on travis

Rails is running against jruby-9.1.15.0 and travis is failing because it
can't seem to install the version it's looking for:

Ref: https://travis-ci.org/rails/rails-dom-testing/jobs/317542357

Since Rails is set to a specific version, this sets rails-dom-testing to
the same version.

.travis.yml

@@ -8,7 +8,7 @@ rvm:
   - 2.1
   - 2.2
   - 2.3.3
-  - jruby
+  - jruby-9.1.15.0
   - ruby-head
 
 matrix: