docs: update release date for 6.12.0

Pushing back a week to fix some bugs that have been found.

- Engine check warnings get written to package.json, resulting in weird
  behavior later.
- GitLab URL parsing needs a bit more tweaks to handle urls like
  /user/repo/repository/ref/archive.tar.gz

CHANGELOG.md

@@ -1,4 +1,4 @@
-## 6.12.0 (2019-10-01):
+## 6.12.0 (2019-10-08):
 
 Now `npm ci` runs prepare scripts for git dependencies, and respects the
 `--no-optional` argument.