Bump spring from 2.0.2 to 2.1.0

Bumps [spring](https://github.com/rails/spring) from 2.0.2 to 2.1.0.
- [Release notes](https://github.com/rails/spring/releases)
- [Changelog](https://github.com/rails/spring/blob/master/CHANGELOG.md)
- [Commits](rails/spring@v2.0.2...v2.1.0)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -266,8 +266,7 @@ GEM
       childprocess (~> 0.5)
       rubyzip (~> 1.2, >= 1.2.2)
     slack-notifier (2.3.2)
-    spring (2.0.2)
-      activesupport (>= 4.2)
+    spring (2.1.0)
     spring-watcher-listen (2.0.1)
       listen (>= 2.7, < 4.0)
       spring (>= 1.2, < 3.0)