Update Travis CI configuration

- Run on xenial distribution
- Stop unsetting _JAVA_OPTIONS. This value is not set in VM-based
  builds
- Always use latest micro versons of JRuby 9.1 and 9.2
- Set add-opens for JRuby 9.1 via JAVA_OPTS. These are needed to avoid
  Java access errors
- Stop building on Rubinius: We don't support it and we never look at
  the build results

.travis.yml

@@ -1,34 +1,40 @@
-dist: trusty
-cache: bundler
 language: ruby
+
+dist: xenial
+
+cache:
+  bundler: true
+
 bundler_args: --without debugging
-before_script:
-  # Avoid Java announcing _JAVA_OPTIONS environment variable
-  # See https://github.com/travis-ci/travis-ci/issues/8408
-  - unset _JAVA_OPTIONS
+
 script: bundle exec rake ci
+
 rvm:
   - 2.3
   - 2.4
   - 2.5
   - 2.6
-  - jruby-9.1.17.0
-  - jruby-9.2.6.0
+  - jruby-9.2
   - jruby-head
   - ruby-head
-  - rubinius-3
+
 matrix:
   allow_failures:
     - rvm: jruby-head
     - rvm: ruby-head
-    - rvm: rubinius-3
+  include:
+    - rvm: jruby-9.1
+      env: "JAVA_OPTS='--add-opens java.base/java.io=jruby --add-opens java.base/sun.nio.ch=jruby'"
+
   fast_finish: true
+
 notifications:
   email:
     - [email protected]
     - [email protected]
     - [email protected]
   irc: irc.freenode.org#reek
+
 branches:
   only:
     - master