Extract graph info from Gemfile.lock [gemnasium only]

What does this MR do?

• Change Gemfile.lock parser to return parser.Dependencies.
• Update expected Dependency Scanning with graph info generated for Bundler projects.

What are the relevant issue numbers?

Report vulnerable dependency paths for Bundler ... (gitlab-org/gitlab#229812)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer