Bump once_cell from 1.3.1 to 1.4.0

Bumps [once_cell](https://github.com/matklad/once_cell) from 1.3.1 to 1.4.0.
- [Release notes](https://github.com/matklad/once_cell/releases)
- [Changelog](https://github.com/matklad/once_cell/blob/master/CHANGELOG.md)
- [Commits](https://github.com/matklad/once_cell/compare/v1.3.1...v1.4.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Cargo.lock

@@ -990,9 +990,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.3.1"
+version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b1c601810575c99596d4afc46f78a678c80105117c379eb3650cf99b8a21ce5b"
+checksum = "0b631f7e854af39a1739f401cf34a8a013dfe09eac4fa4dba91e9768bd28168d"
 
 [[package]]
 name = "opaque-debug"

zebrad/Cargo.toml

@@ -33,4 +33,4 @@ zebra-network = { path = "../zebra-network" }
 
 [dev-dependencies]
 abscissa_core = { version = "0.5", features = ["testing"] }
-once_cell = "1.3"
+once_cell = "1.4"