Update jruby version

The jruby 9.2 builds were failing during `bundle install` with the following error:

```
bundle install --jobs=3 --retry=3 --path=${BUNDLE_PATH:-vendor/bundle}
256Error occurred during initialization of boot layer
257java.lang.module.FindException: Unable to derive module descriptor for /home/travis/.rvm/rubies/jruby-9.2.0.0/lib/jruby.jar
258Caused by: java.lang.module.InvalidModuleDescriptorException: Test.class found in top-level directory (unnamed package not allowed in module)
```

After updating jruby to 9.2.7, the builds started failing with the
following:

```
You must use Bundler 2 or greater with this lockfile.
349The command "bin/test" exited with 20.
```

Everything worked correctly after updating Bundler inside the
`before_install` hook.

Ref: https://travis-ci.community/t/unable-to-test-against-jruby-9-2-7-0/3138

.travis.yml

@@ -8,7 +8,7 @@ rvm:
   - 2.6.3
   - ruby-head
   - jruby-9.1.17.0
-  - jruby-9.2.0.0
+  - jruby-9.2.7.0
 gemfile:
   - gemfiles/Gemfile.rails-5-0-stable
   - gemfiles/Gemfile.rails-5-1-stable
@@ -26,8 +26,10 @@ matrix:
     - rvm: ruby-head
     - rvm: jruby-9.1.17.0
       gemfile: Gemfile
-    - rvm: jruby-9.2.0.0
+    - rvm: jruby-9.2.7.0
       gemfile: Gemfile
+before_install:
+  - gem update --system
 notifications:
   email: false
   slack: