remove gemnasium batch from README. [ci skip]

The rails/rails gemnasium page (https://gemnasium.com/rails/rails) is not helping much.
It lists 0 dependencies so there is not much to track. Also our gems mostly depend
on other gems we control. There is not much point in monitoring them.

The batch was introduced with #3909.
Remove obsolete line (was needed for Ruby 1.8.7 support)
Remove obsolete test (builtin group_by is now used)
Strengthen test with different nb of rows and columns
Return sized enumerator from Batches#find_in_batches
Return sized enumerator from Batches#find_each
Return sized enumerator from Enumerable#index_by
Merge pull request #13938 from marcandre/sized_enumerator

Sized enumerator
fix interplay of humanize and html_escape
Merge pull request #13953 from seanwalbran/fix-humanize-match-reference

fix interplay of humanize and html_escape
Fixed "Hash#to_param confused by empty hash values #13892"
Move test to the right file
Test with a blank value
Refatoring the method to avoid shot-circuit return
Add CHANGELOG entry

Closes #13909
Fix to_query with empty arrays too
update contribution doc grammar. [ci skip]

- Manually applied from #13951.
- Discussion at #13947.
- Removed trailing whitespace from https://github.com/dpmehta02/rails/commit/18044e86af93672dfc38befbe974261e87b3518d

/cc @dpmehta02
synchronize 4.1 release notes with CHANGELOGS. [ci skip]

/cc @chancancode
docs, Associations also raise on name conflicts. [ci skip]

Follow up to https://github.com/rails/rails/commit/580f0b61dc99c6854fa930a761d28a3ab08163f7#commitcomment-5293470
Add config to disable schema dump after migration

* Add a config on Active Record named `dump_schema_after_migration`
* Schema dump doesn't happen if the config is set to false
* Set default value of the config to true
* Set config in generated production environment file to false
* Update configuration guide
* Update CHANGELOG