Upgrade mail and nokogiri gems due to security issues

What does this MR do?

Fixes security issues in gems as reported on Gemnasium:

• CVE-2015-9097
• CVE-2016-4658
• CVE-2017-5029

Are there points in the code the reviewer needs to double check?

Some other gems had to be updated as well to satisfy the dependency requirements, I checked their CHANGELOGs and couldn't see any breaking changes.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13588