Bump chartkick from 5.0.1 to 5.1.2

Bumps [chartkick](https://github.com/ankane/chartkick) from 5.0.1 to 5.1.2.
- [Changelog](https://github.com/ankane/chartkick/blob/master/CHANGELOG.md)
- [Commits](ankane/chartkick@v5.0.1...v5.1.2)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -138,7 +138,7 @@ GEM
       xpath (~> 3.2)
     case_transform (0.2)
       activesupport
-    chartkick (5.0.1)
+    chartkick (5.1.2)
     coderay (1.1.3)
     concurrent-ruby (1.3.4)
     connection_pool (2.3.0)