Don't run specs with jruby 9.1 and Rails 6

It's not supported due to zeitwerk

```
zeitwerk-2.1.9 requires ruby version >= 2.4.4, which is incompatible with the
287current version, ruby 2.3.3p0 (jruby 9.1.17.0)
```

Author: tegon

.travis.yml

@@ -20,6 +20,8 @@ matrix:
       gemfile: Gemfile
     - rvm: 2.4.6
       gemfile: Gemfile
+    - rvm: jruby-9.1.17.0
+      gemfile: Gemfile
   allow_failures:
     - rvm: ruby-head
     - rvm: jruby-9.1.17.0