Upgrade rubocop to 0.49.1 and rubocop-rspec to 1.15.1

Upgrade 2 gems:

• rubocop from 0.47.1 to 0.49.1
  • fixes CVE-2017-8418 (CVSS v3: 3.3)
  • changelog: https://github.com/bbatsov/rubocop/compare/v0.47.1...v0.49.1#diff-4ac32a78649ca5bdd8e0ba38b7006a1e
  • all changes: https://github.com/bbatsov/rubocop/compare/v0.47.1...v0.49.1
• rubocop-rspec from 1.15.0 to 1.15.1
  • changelog: https://github.com/backus/rubocop-rspec/releases/tag/v1.15.1

/label gem update backstage backend