Preparing for 4.2.0.beta1 release
Use system /tmp when testing actionpack

https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
added support and enabled parallel execution of the actionpack tests.

However it introduced https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
since one cannot connect to a socket file that's inside a Vagrant synced folder
due to security restrictions, and DRb tries to.

Also rename the temporary files to make it obvious that they're rails-related,
since now they're placed outside the project's directory.

Fixes https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
use self instead of #read_attribute
Don't test PostgreSQL implementation details

Asserting that the message contains "QUERY PLAN" is enough for the adapter's test.
The plan may vary according to number of tuples etc. but that's out of our concern.
Some documentation fixes [ci skip]
Make test:isolated run without bundler for Action Mailer

Action Mailer tests weren't able to run in isolation without the bundle
exec prefix since we were requiring gems before requiring abstract_unit.

We don't need the `gem` call thus and the require_relative since the
test directory should be present in the load path when we run any test.
Avoid skipping Sidekiq tests on Rubinius

Now that Travis provides support for Rubinius 2.2.10, we can run the
Sidekiq tests against this version.
Merge pull request #16742 from robin850/rbx-aj-sidekiq

Avoid skipping Sidekiq tests on Rubinius
Merge pull request #16741 from robin850/am-isolated-wo-bundle

Make test:isolated run without bundler for Action Mailer
Don't allow AR::Tasks::DatabaseTasks.migrate to mutate Migration.verbose value

This fixes random test failure in migrator_test.rb
Fixed Active Job Backburner adapter
Merge pull request #16746 from cristianbica/fix-backburner-adapter

Fixed Active Job Backburner adapter
Merge branch 'master' of github.com:rails/rails
Added yield to Object#presence
Update examples to show real worth
Use instance_eval on @tenderlove's suggestion :trollface:
Clarify the origin of this great addition to Rails :trollface: :trollface :trollface:
We tenderized the wrong method! Object#try already had the yield option, just needed some tenderloving instance_eval to fit the bill
reduce object allocations in utc_offset

`try` allocates an array on every call, we should avoid calling it in
hotspots.  This reduced AttributeMethodsTest#test_setting_time_zone_aware_attribute_with_string
from 18k allocations to 14k
Forgot to set Migration.verbose at f8d9a51949d6f6cfcab7cc7d501e137836eb88dc :bomb: