Rails Upgrade to 6

[xinyis991105]

Description

This PR upgrades rails to 6 along with the necessary gems.

Motivation and Context

Keep up with the current latest rails.

How Has This Been Tested?

1. Delete Gemfile.lock. Run bundle install.
2. Drop the current database and re-migrate.
3. Go through the behavioral testing plan.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting
• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

Other issues / help required

If unsure, feel free to submit first and we'll help you along.

[xinyis991105]

linting is done and a reminder has been added in another commit

[fanpu]

LGTM, thanks Xinyi!