Update JRuby version run by Travis

We recently started getting an error in Travis for our JRuby tests.
Upgrading the JRuby version seems to fix it

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

Author: Daniel Magliola

.travis.yml

@@ -9,4 +9,4 @@ rvm:
   - 2.4.5
   - 2.5.3
   - 2.6.0
-  - jruby-9.1.5.0
+  - jruby-9.1.9.0