Bump splinter from 0.14.0 to 0.15.0

Bumps [splinter](https://github.com/cobrateam/splinter) from 0.14.0 to 0.15.0.
- [Release notes](https://github.com/cobrateam/splinter/releases)
- [Changelog](https://github.com/cobrateam/splinter/blob/master/docs/news.rst)
- [Commits](https://github.com/cobrateam/splinter/compare/0.14.0...0.15.0

)

---
updated-dependencies:
- dependency-name: splinter
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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