Build(deps): Bump css_parser from 1.14.0 to 1.15.0

Bumps [css_parser](https://github.com/premailer/css_parser) from 1.14.0 to 1.15.0.
- [Changelog](https://github.com/premailer/css_parser/blob/master/CHANGELOG.md)
- [Commits](premailer/css_parser@v1.14.0...v1.15.0)

---
updated-dependencies:
- dependency-name: css_parser
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -122,7 +122,7 @@ GEM
     connection_pool (2.4.1)
     content_disposition (1.0.0)
     crass (1.0.6)
-    css_parser (1.14.0)
+    css_parser (1.15.0)
       addressable
     date (3.3.3)
     diff-lcs (1.5.0)