Auto merge of #15526 - servo:1.15, r=Manishearth

Upgrade to Rust 1.15.1 for build-geckolib

Firefox CI already does:

https://bugzilla.mozilla.org/show_bug.cgi?id=1338311
https://bugzilla.mozilla.org/show_bug.cgi?id=1337153

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/15526)
<!-- Reviewable:end -->

rust-stable-version

@@ -1 +1 @@
-1.13.0
+1.15.1