Run latest precompiled JRuby on CI only against ActionPack

Uses latest precompiled JRuby so that
we don't spend time downloading versions Travis has
not already compiled. http://rubies.travis-ci.org/

Uses latest jdk: oraclejdk8
per
https://docs.travis-ci.com/user/build-environment-updates/2015-02-03/#Ruby-VM
and
https://docs.travis-ci.com/user/languages/ruby/#Supported-Ruby-Versions-and-RVM

Follows on work in https://github.com/rails/rails/pull/23927 which was reverted
26fe5fa08d

JRUBY_OPTS minimize GC, disable JIT, for max test speed
  - https://github.com/rails/rails/pull/16613
  - https://github.com/rails/rails/pull/17088

Have Rails use JRuby-compatible Rake 11.1
  - The Rake task was passing --verbose, an invalid option, to contemporary JRuby
  - https://github.com/ruby/rake/pull/120
  - https://github.com/rails-api/active_model_serializers/pull/1585
  - https://github.com/jruby/jruby/issues/3653#issuecomment-195883717

No advantage to directly mounting JRuby over installing from cache; both on S3
  - b2d5b336b5
  - f4fad041b2

.travis.yml

@@ -29,6 +29,13 @@ rvm:
   - 2.3.0
   - ruby-head
 matrix:
+  include:
+    # Latest compiled version in http://rubies.travis-ci.org
+    - rvm: jruby-9.0.5.0
+      jdk: oraclejdk8
+      env:
+        - "JRUBY_OPTS='--dev -J-Xmx1024M'"
+        - "GEM='ap'"
   allow_failures:
     - rvm: ruby-head
   fast_finish: true

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gemspec
 
 # We need a newish Rake since Active Job sets its test tasks' descriptions.
-gem 'rake', '>= 10.3'
+gem 'rake', '>= 11.1'
 
 # This needs to be with require false to ensure correct loading order, as it has to
 # be loaded after loading the test library.

Gemfile.lock

@@ -197,7 +197,7 @@ GEM
       rails-deprecated_sanitizer (>= 1.0.1)
     rails-html-sanitizer (1.0.3)
       loofah (~> 2.0)
-    rake (10.5.0)
+    rake (11.1.1)
     rb-fsevent (0.9.7)
     rb-inotify (0.9.5)
       ffi (>= 0.5.0)