Increase gemnasium-maven MAX_IMAGE_SIZE_MB

What does this MR do?

In response to recurring failures in our daily image builds, this merge request proposes an increase in the maximum allowed image size.

What are the relevant issue numbers?

Composition Analysis - Reaction Rotation for 16.10 (gitlab-org/secure/general#297 - closed) • Philip Cunningham, Nick Ilieskou • 16.10

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