Update activerecord requirement from ~> 4.0 to ~> 6.0

[dependabot-preview]

Updates the requirements on activerecord to permit the latest version.

Changelog

Sourced from activerecord's changelog.

Rails 6.0.0 (August 16, 2019)

• Preserve user supplied joins order as much as possible.

  Fixes #36761, #34328, #24281, #12953.

  Ryuta Kamizono

• Make the DATABASE_URL env variable only affect the primary connection. Add new env variables for multiple databases.

  John Crepezzi, Eileen Uchitelle

• Add a warning for enum elements with 'not_' prefix.

  class Foo
    enum status: [:sent, :not_sent]
  end
  

  Edu Depetris

• Make currency symbols optional for money column type in PostgreSQL

  Joel Schneider

Rails 6.0.0.rc2 (July 22, 2019)

• Add database_exists? method to connection adapters to check if a database exists.

  Guilherme Mansur

• PostgreSQL: Fix GROUP BY with ORDER BY virtual count attribute.

  Fixes #36022.

  Ryuta Kamizono

• Make ActiveRecord ConnectionPool.connections method thread-safe.

  Fixes #36465.

  Jeff Doering

• Fix sqlite3 collation parsing when using decimal columns.

  Martin R. Schuster

• Fix invalid schema when primary key column has a comment.

  Fixes #29966.

... (truncated)

Commits

• 66cabed Preparing for 6.0.0 release
• f63df2b Merge pull request #36949 from 97jaz/thread-local-prepared-statements
• 92784ad Nodoc class that should not be part of public API
• 4f912de Merge pull request #36873 from jhawthorn/schema_sha_parallel
• 0169c93 Merge pull request #36883 from jhawthorn/schema_sha_fix
• c550e71 Merge pull request #36870 from jhawthorn/schema_sha
• 26ab0ad Merge pull request #36848 from jhawthorn/type_error_on_resolve_connection
• 80c1f15 Restore an ability that reorder with first taking non-deterministic result
• eae97cd Fix typo in test name
• c5b68fb Merge pull request #36863 from okuramasafumi/add-explanation-of-context-to-ac...
• Additional commits viewable in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot recreate will recreate this PR, overwriting any edits that have been made to it
• @dependabot merge will merge this PR after your CI passes on it
• @dependabot squash and merge will squash and merge this PR after your CI passes on it
• @dependabot cancel merge will cancel a previously requested merge and block automerging
• @dependabot reopen will reopen this PR if it is closed
• @dependabot close will close this PR and stop Dependabot recreating it. You can achieve the same result by closing it manually
• @dependabot ignore this major version will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this minor version will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)
• @dependabot use these labels will set the current labels as the default for future PRs for this repo and language
• @dependabot use these reviewers will set the current reviewers as the default for future PRs for this repo and language
• @dependabot use these assignees will set the current assignees as the default for future PRs for this repo and language
• @dependabot use this milestone will set the current milestone as the default for future PRs for this repo and language
• @dependabot badge me will comment on this PR with code to add a "Dependabot enabled" badge to your readme

Additionally, you can set the following in your Dependabot dashboard:

• Update frequency (including time of day and day of week)
• Pull request limits (per update run and/or open at any time)
• Out-of-range updates (receive only lockfile updates, if desired)
• Security updates (receive only security updates, if desired)

Finally, you can contact us by mentioning @dependabot.

[dependabot-preview]

Looks like activerecord is up-to-date now, so this is no longer needed.