build(deps): bump petgraph from 0.5.0 to 0.5.1

Bumps [petgraph](https://github.com/petgraph/petgraph) from 0.5.0 to 0.5.1.
- [Release notes](https://github.com/petgraph/petgraph/releases)
- [Commits](petgraph/petgraph@0.5.0...0.5.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

Cargo.lock

@@ -44,7 +44,7 @@ strum_macros = "0.18.0"
 slog-async = "2.5.0"
 slog-envlogger = "2.1.0"
 slog-term = "2.5.0"
-petgraph = "0.5.0"
+petgraph = "0.5.1"
 tiny-keccak = "1.5.0"
 tokio = { version = "0.2.21", features = ["stream", "rt-threaded", "rt-util", "blocking", "time", "sync", "macros", "test-util"] }
 tokio-retry = { git = "https://github.com/graphprotocol/rust-tokio-retry", branch = "update-to-tokio-02" }