Pure rack apps can be mounted with a name

See https://github.com/rails/rails/commit/9b15828b5c347395b42066a588c88e5eb4e72279#commitcomment-8764492
Bump ruby version to 2.1.0 in rails.gemspec too.
Merge pull request #17846 from biow0lf/bump-ruby-version

Bump ruby version to 2.1.0 in rails.gemspec too.
Fix link in documentation. [ci skip]
Merge pull request #17848 from biow0lf/fix-link-in-action_view-base

Fix link in documentation. [ci skip]
Merge pull request #17823 from byroot/fix-mount-rack-apps-with-as

Pure rack apps can be mounted with a name
Stop using `Arel::Table.engine`

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.
Use arel master
Update Arel usage for rails/arel#98fc259

`where_sql` now requires that we pass it an engine. None of the manager
classes take an engine in their constructor.
[ci skip] fix description of url_for
Merge pull request #17850 from y-yagi/patch-2

[ci skip] fix description of url_for