package/nodejs: bump version to v8.15.0

package/nodejs: bump version to v8.15.0

Fixes regressions introduced by the v8.14.0 security release.  From the
announcement:

The 8.14.0 security release introduced some unexpected breakages on the 8.x
release line.  This is a special release to fix a regression in the HTTP
binary upgrade response body and add a missing CLI flag to adjust the max
header size of the http parser.

https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V8.md#8.15.0

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>