build: bump backports from 3.14.0 to 3.15.0 in /Library/Homebrew

Bumps [backports](https://github.com/marcandre/backports) from 3.14.0 to 3.15.0.
- [Release notes](https://github.com/marcandre/backports/releases)
- [Changelog](https://github.com/marcandre/backports/blob/master/CHANGELOG.rdoc)
- [Commits](https://github.com/marcandre/backports/compare/v3.14.0...v3.15.0

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Library/Homebrew/Gemfile.lock

@@ -7,7 +7,7 @@ GEM
       minitest (~> 5.1)
       tzinfo (~> 1.1)
     ast (2.4.0)
-    backports (3.14.0)
+    backports (3.15.0)
     concurrent-ruby (1.1.5)
     connection_pool (2.2.2)
     coveralls (0.8.23)