Bump backtrace and rustc-demangle

This commit bumps a few additional packages that could be related to
the segfaults seen in production.  See #1805 for more info.

`cargo update -p backtrace -p rustc-demangle`

```
Updating backtrace v0.3.3 -> v0.3.38
Updating backtrace-sys v0.1.16 -> v0.1.31
Removing dbghelp-sys v0.2.0
Updating rustc-demangle v0.1.5 -> v0.1.16
```

Author: jtgeibel