Update generated spec_helper.

- Add new `include_chain_clauses_in_custom_matcher_descriptions`
  config option. It will default to `true` in RSpec 4 so we
  have not put it in the commented-out section.
- Move `verify_partial_doubles` config option into non-commented
  out section as well. I want to default it to `true` in RSpec 4
  as well.
- Switch from `syntax` config options to `disable_monkey_patching!`

Fixes #1645.
Add changelog entry.

[ci skip]
Merge pull request #1647 from rspec/update-generated-config-for-rspec-3-1

Update generated spec_helper.