Bump concurrent-ruby from 1.3.5 to 1.3.7 (#61)

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.3.5 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@v1.3.5...v1.3.7)

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

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Author: dependabot[bot]

Gemfile.lock

@@ -21,7 +21,7 @@ GEM
       sass (>= 3.3.0, < 3.5)
     compass-import-once (1.0.5)
       sass (>= 3.2, < 3.5)
-    concurrent-ruby (1.3.5)
+    concurrent-ruby (1.3.7)
     csv (3.3.2)
     em-websocket (0.5.3)
       eventmachine (>= 0.12.9)