Bump chartkick from 5.1.4 to 5.1.5

Bumps [chartkick](https://github.com/ankane/chartkick) from 5.1.4 to 5.1.5.
- [Changelog](https://github.com/ankane/chartkick/blob/master/CHANGELOG.md)
- [Commits](ankane/chartkick@v5.1.4...v5.1.5)

---
updated-dependencies:
- dependency-name: chartkick
  dependency-version: 5.1.5
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -147,7 +147,7 @@ GEM
       xpath (~> 3.2)
     case_transform (0.2)
       activesupport
-    chartkick (5.1.4)
+    chartkick (5.1.5)
     childprocess (5.1.0)
       logger (~> 1.5)
     coderay (1.1.3)