Revert "Install bundle gems to the system not to vendor/bundle"

This reverts commit 875dd3edfd34eaa20120b6589b0d1905230e948f.

The issue was fixed in rbx-2.2.3
Remove deprecation warning for FIXTURES_PATH

FIXTURES_PATH has a bit different case than DatabaseTasks.fixtures_path,
which was added along with deprecation. A use case for FIXTURES_PATH
could be loading fixtures from a different directory just for a given
test run. The implementation is fairly simple, so leaving it as it is
won't hurt.
Add a missing changelog entry for #13534 [ci skip]
Merge pull request #13538 from robin850/patch-14

Add a missing changelog entry for #13534 [ci skip]
tidy AR CHANGELOG. [ci skip]
Update explanation to match example
Merge pull request #13539 from StevenNunez/patch-1

Update explanation to match example [ci skip]
Merge pull request #13536 from schneems/schneems/connection-spec-docs

[ci skip] ConnectionSpecification::Resolver Docs
Allow "url" sub key in database.yml configuration

Currently a developer can pass in a YAML configuration that fully specifies connection information:

```
production:
  database: triage_production
  adapter: password
  pool: 5
```

They can also pass in a string that specifies a connection URL directly to an environment key:

```
production: postgresql://localhost/foo
```

This PR allows the use of both a connection url and specifying connection attributes via YAML through the use of the "url" sub key:

```
production:
  url: postgresql://localhost/foo
  pool: 3
```

This will allow developers to inherit Active Record options such as `pool` from `&defaults` and still use a secure connection url such as `<%= ENV['DATABASE_URL'] %>`. The URL is expanded into a hash and then merged back into the YAML hash. If there are any conflicts, the values from the connection URL are preferred. 

Talked this over with @josevalim