Merge pull request #16 from mwatts/dependabot/cargo/petgraph-approx-0.6

Update petgraph requirement from ~0.5 to ~0.6

db/Cargo.toml

@@ -16,7 +16,7 @@ lazy_static = "~1.4"
 log = "~0.4"
 ordered-float = "~2.5"
 time = "~0.2"
-petgraph = "~0.5"
+petgraph = "~0.6"
 serde = { version = "~1.0", optional = true }
 serde_json = { version = "~1.0", optional = true }
 serde_derive = { version = "~1.0", optional = true }