Use latest JRuby release in Travis CI build matrix

JRuby v9.1.15.0 was released on 07 Dec 2017 [1].

[1]: http://jruby.org/2017/12/07/jruby-9-1-15-0.html

.travis.yml

@@ -13,14 +13,14 @@ rvm:
   - 2.4.3
   - 2.5.0
   - ruby-head
-  - jruby-9.1.9.0
+  - jruby-9.1.15.0
 gemfile:
   - Gemfile
 env:
   - MOCHA_OPTIONS=debug
 matrix:
   include:
-    - rvm: jruby-9.1.9.0
+    - rvm: jruby-9.1.15.0
       gemfile: gemfiles/Gemfile.minitest.latest
       env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
     - rvm: ruby-head
@@ -92,7 +92,7 @@ matrix:
     - rvm: 1.8.7-p371
       gemfile: gemfiles/Gemfile.minitest.1.3.0
       env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=minitest
-    - rvm: jruby-9.1.9.0
+    - rvm: jruby-9.1.15.0
       gemfile: gemfiles/Gemfile.test-unit.latest
       env: MOCHA_OPTIONS=debug MOCHA_RUN_INTEGRATION_TESTS=test-unit
     - rvm: ruby-head