fix(deps): update dependency gatsby to v2.19.15

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby	dependencies	patch	2.19.14 -> 2.19.15

---

Release Notes

gatsbyjs/gatsby

### [`v2.19.15`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.19.14...gatsby@2.19.15) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.19.14...gatsby@2.19.15)

---

Renovate configuration

:date: Schedule: At any time (no schedule defined).

:vertical_traffic_light: Automerge: Enabled.

:recycle: Rebasing: Whenever PR becomes conflicted, or if you tick the rebase/retry checkbox below.

:no_bell: Ignore: Close this PR and you won't be reminded about this update again.

---

• [ ] If you want to rebase/retry this PR, check this box

---

This PR has been generated by WhiteSource Renovate. View repository job log here.