[ci skip] remove guide section on models in migrations [ci skip]

Both examples given are incorrect, and no one can seem to come up with
a realistic situation where this is an actual problem. For now, it seems like
the best thing to do is just scrap the section altogether.

This test currently outputs the following warning:

```
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"   ORDER BY "postgresql_enums"."id" ASC LIMIT 1)
unknown OID: current_mood(3567879) (SELECT  "postgresql_enums".* FROM "postgresql_enums"  WHERE "postgresql_enums"."id" = $1 LIMIT 1)
```

We have an open PR to deal with this issue. It will dynamically
register the OID for enum columns. This test case is merely to exhibit
the current behavior of PostgreSQL enum columns.

Bump version of bcrypt gem

From #14026:

    Specific rbx-2 to limit testing on Rubinius 2.x (since there will be
    other versions of Rubinius > 2.x soon).

    Also, as of Rubinius 2.2.5, it is no longer necessary to bundle the
    rubysl gem.

This is what Rails master/4.1 supports, so we don't need to add rubysl to
gemfiles anymore.

`before_add` callbacks are fired before the record is saved on
`has_and_belongs_to_many` assocations *and* on `has_many :through`
associations.  Before this change, `before_add` callbacks would be fired
before the record was saved on `has_and_belongs_to_many` associations, but
*not* on `has_many :through` associations.

Fixes #14144

Fix test name typo in app generator tests.

Conflicts:
	activerecord/CHANGELOG.md

travis s/2.1.0/2.1.1

Handle missing environment from non empty config

Skip test_migrate_revert_add_index_with_name for some databases

do not allow to create duplicate indexes on the same columns

We noticed the rails gem increased in size between the beta and rc
versions, and the reason is that some guides images were updated but not
properly optimized, and one image changed from png to jpg, way bigger.

Converting the jpg image back to png, and using ImageOptim to optimize
all the images for web view, improves the situation [ci skip].

---

In current master/4-1-stable:

    508K  guides/assets/images/getting_started/

In v4.1.0.beta1:

    268K  guides/assets/images/getting_started/

In 4-0-stable

    328K  guides/assets/images/getting_started/

After the patch:

    260K  guides/assets/images/getting_started/

Add missing parantheses in index_exists?

Remove 4-1 related entries from master [ci skip]

See 449039a8 for the original commit.

Sungularize ActionController::UnpermittedParameters error in case when only 1 parameter is unpermitted.

Standardized punctuation.

[skip ci] Fixing typo in documentation: actionview/lib/action_view/helpers/url_helper.rb

This is a follow up to #14170. While backporting I recognized
that this call is not needed at all.

there could be more than 10 nested records

Go with #presence_in instead of #present_in -- it doesnt sound quite as good, but the closer relationship to #presence over #present is ultimately worth it

Simple Sungularize ActionController::UnpermittedParameters error in case when only 1 parameter is unpermitted.

This is a follow up to #13515. It includes the name given and
the singularized version in the warning message. This will aide the user
to see wether the detected singular was right or not.

coerce strings when reading attributes

Extract local cache middleware

Extract LocalCache Middleware, so it can requires rack dependencies,
without adding rack dependencies to `AS::Cache::Strategy::LocalCache`.