Transitive dependencies of `rouille` incompatible with future versions of Rust

```
integration-test $ cargo clippy --all-targets
Checking integration-test v0.0.0 (/Users/emorley/src/heroku-java-metrics-agent/integration-test)
Finished dev [unoptimized + debuginfo] target(s) in 0.19s
warning: the following packages contain code that will be rejected by a future version of Rust: buf_redux v0.8.4, multipart v0.18.0
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1
```

These are transitive dependencies of `rouille`:
https://github.com/tomaka/rouille/issues/271

Start by running `cargo clippy --all-targets` in `integration-test` and inspect the transitive dependency chain from `rouille` to `buf_redux` and `multipart`. Review the linked `rouille` issue, then verify that the future-incompatibility warning is resolved without breaking the integration test build.

Written by the indexing model from the issue text.