Bump concurrent-ruby from 1.3.4 to 1.3.7 in /examples/simple_rails_api

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.3.4 to 1.3.7.
- [Release notes](https://github.com/ruby-concurrency/concurrent-ruby/releases)
- [Changelog](https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md)
- [Commits](ruby-concurrency/concurrent-ruby@v1.3.4...v1.3.7)

---
updated-dependencies:
- dependency-name: concurrent-ruby
  dependency-version: 1.3.7
  dependency-type: indirect
...

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

Author: dependabot[bot]

examples/simple_rails_api/Gemfile.lock

@@ -64,7 +64,7 @@ GEM
       msgpack (~> 1.0)
     builder (3.3.0)
     byebug (11.1.3)
-    concurrent-ruby (1.3.4)
+    concurrent-ruby (1.3.7)
     crass (1.0.6)
     date (3.3.4)
     erubi (1.13.0)