Bump Ruby on Rails to 5.0.7.1

What does this MR do?

Bumps Ruby on Rails framework to 5.0.7.1.

Fix the CVE-2018-16476 vulnerability.

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/gl-security/engineering/issues/296

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.