Auto merge of #2488 - jtgeibel:update/rustc-and-conduit-hyper-deps, r=JohnTitor

Bump rustc and some low-level dependencies

This bumps to the latest stable rustc on Heroku and does an aggressive update of `conduit-hyper`, `web-sys`, and `reqwest`. I have coupled these updates so that we clean the build cache once while also updating some low-level dependencies. In particular, this bumps crates like `libc`, `cc`, and `serde` that would result in large rebuilds anyway.

I believe it is safe to couple these updates because these crates have been bumped in the recent past without issue. See #2213 and #2246. The commit message show details of the updated versions in each step.

r? @JohnTitor

Author: bors