Update `xctool` to v0.1.15

Summary: Bumped `xctool` for a variety of reasons: https://github.com/facebook/xctool/releases/tag/v0.1.15

Test Plan: None.

Reviewers: ablackwood, gbeaman

Reviewed By: gbeaman

Subscribers: mbishop

Differential Revision: https://devportal.imobile3.com/D9225