servo: Revert "Update Rust stable to 1.17.0"

This reverts commit 8a9d9ab2f9d7d384ac0e94671fae705088c26399.

Source-Repo: https://github.com/servo/servo
Source-Revision: befb017f7f8b6d6387c3804dbf2f493cc297b85d

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : e9aa67d879f8188428a81821250f58888898efc2

servo/rust-stable-version

@@ -1 +1 @@
-1.17.0
+1.16.0