CI: Configure JRuby to update in 1 location

Easier to set the version in fewer places.

Author: olleolleolle

.travis.yml

@@ -9,11 +9,11 @@ matrix:
     # Latest versions first
     - name: MRI 2.7.0 Latest
       rvm: 2.7.0
-    - name: JRuby 9.2.16.0 Latest on Java 11
-      rvm: jruby-9.2.16.0
+    - name: Latest JRuby on Java 11
+      rvm: jruby-9.2.18.0
       jdk: oraclejdk11
-    - name: JRuby 9.2.16.0 Latest on Java 8
-      rvm: jruby-9.2.16.0
+    - name: Latest JRuby on Java 8
+      rvm: jruby-9.2.18.0
       jdk: openjdk8
     - name: TruffleRuby Latest
       rvm: truffleruby