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.
Fix --pattern so it stil applies when `files_to_run` is accessed early.

Related to #1589.
Fix `files_to_run` so that setting `pattern` after it was already accessed.
Merge pull request #1652 from rspec/fix-pattern-for-generated-spec-helper

Fix --pattern so it stil applies when `files_to_run` is accessed early.
Exclude exe/rspec from backtraces.

When users had installed rspec-core as a bundler :git dependency,
the rspec binary (locally defined in `exe/rspec`) would be included
in backtraces. This wouldn't affect users who had done a normal
gem install since rubygems copies it to a `bin` directory.

Fixes #1646.
Only ignore JRuby files on JRuby.

This should make things slightly faster on MRI as it’s
one less regex to check.
Remove unused file.
Update changelog.