Don't parallelize tests running on jruby

Running tests using Minitest's `parallelize_me!` leads to the following
error:

```ruby

ArgumentError: marshal data too short
        load at org/jruby/RubyMarshal.java:150
         run at
/home/travis/build/rails/rails/activesupport/lib/active_support/testing/isolation.rb:82
  _run_suite at
/home/travis/build/rails/rails/vendor/bundle/jruby/1.9/gems/minitest-4.7.5/lib/minitest/unit.rb:933
        each at
/home/travis/build/rails/rails/vendor/bundle/jruby/1.9/gems/minitest-4.7.5/lib/minitest/parallel_each.rb:45
rake aborted!
```

Full Log here:
https://s3.amazonaws.com/archive.travis-ci.org/jobs/16812844/log.txt

Above Error Occurs in many railties, ap, amo, av when tests are run on jruby. with this patch, atleast the tests would compete & not due due weird threading issues. 


#11700