Bump simplecov from 0.18.3 to 0.18.4

Bumps [simplecov](https://github.com/colszowka/simplecov) from 0.18.3 to 0.18.4.
- [Release notes](https://github.com/colszowka/simplecov/releases)
- [Changelog](https://github.com/colszowka/simplecov/blob/master/CHANGELOG.md)
- [Commits](simplecov-ruby/simplecov@v0.18.3...v0.18.4)

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

Gemfile.lock

@@ -333,7 +333,7 @@ GEM
       actionpack (>= 5.0)
       activemodel (>= 5.0)
     simple_oauth (0.3.1)
-    simplecov (0.18.3)
+    simplecov (0.18.4)
       docile (~> 1.1)
       simplecov-html (~> 0.11)
     simplecov-html (0.12.1)