Update JRuby to 9.1.13.0

Author: aycabta

.travis.yml

@@ -10,11 +10,11 @@ rvm:
   - 2.3.4
   - 2.4.1
   - ruby-head
-  - jruby-9.1.12.0
+  - jruby-9.1.13.0
 env:
   global:
     NOBENCHMARK=1
 script: rake
 matrix:
   allow_failures:
-    - rvm: jruby-9.1.12.0
+    - rvm: jruby-9.1.13.0