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](https://github.com/rails/spring/compare/v2.0.2...v2.1.0)

Gemfile.lock

@@ -470,8 +470,7 @@ GEM
       simplecov (>= 0.4.1)
     slack-notifier (2.3.2)
     slop (3.6.0)
-    spring (2.0.2)
-      activesupport (>= 4.2)
+    spring (2.1.0)
     spring-commands-rspec (1.0.4)
       spring (>= 0.9.1)
     spring-commands-testunit (1.0.1)