[ci skip] fix description of url_for

`where_sql` now requires that we pass it an engine. None of the manager
classes take an engine in their constructor.

We never actually make use of it on the table, since we're constructing
the select manager manually. It looks like if we ever actually were
grabbing it from the table, we're grossly misusing it since it's meant
to vary by AR class.

Its existence on `Arel::Table` appears to be purely for convenience
methods that are never used outside of tests. However, in production
code it just complicates construction of the tables on the rails side,
and the plan is to remove it from `Arel::Table` entirely. I'm not
convinced it needs to live on `SelectManager`, etc either.

Pure rack apps can be mounted with a name

Fix link in documentation. [ci skip]

Bump ruby version to 2.1.0 in rails.gemspec too.

See https://github.com/rails/rails/commit/9b15828b5c347395b42066a588c88e5eb4e72279#commitcomment-8764492

Pass symbol as an argument instead of a block

Prevent Numeric#to_s from allocating an array

Bump required Ruby version to 2.1.0

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

Before the exact minimum version is fully decided, @arthurnn [suggests](https://github.com/rails/rails/pull/17830#issuecomment-64940383)
that **at least** version 2.1.0 **must** be required by the `gemspec` files.

Remove a reference to Ruby 1.9 in Guides

[ci skip]

Now that Rails requires Ruby >= 2.0 there is no need to document
Ruby 1.9.

[ci skip] fix class name

fixed indent of end in jdom.rb

Refactor `add_column_options!`, to move the quoting of default value for :uuid in `quote_value`.

delayed_job_active_record doesn't allow Rails 5 yet

We will support only Ruby >= 2.1.

But right now we don't accept pull requests with syntax changes to drop
support to Ruby 1.9.

4.2 release notes pass [skip ci]

Adds preloaded_records method to NullPreloader - fixes #16070

This fixes a regression where preloading association throws an
exception if one of the associations in the preloading hash doesn't
exist for one record.

Fixes #16070

Fixed a bug where AR::RecordNotSaved loses the given error message

Remove ecosystem link from guides

Refactor `SchemaCreation#visit_AddColumn`

Change originated from https://github.com/rails/rails/pull/17739#issuecomment-64829088

/cc @metaskills