Set chrono minimum version to v0.4.21
* Set chrono minimum version to v0.4.20
Per [RUSTSEC-2020-0159](https://rustsec.org/advisories/RUSTSEC-2020-0159.html), `chrono` may segfault under certain circumstances. This bumps `chrono`'s version to `0.4.20` to mitigate the issue.
* Change chrono version to v0.4.21
@@ -28,7 +28,7 @@ bincode = "1"
blake3 = "1"
byteorder = "1.0"
bytes = "1"
-chrono = { version = "0.4", optional = true }
+chrono = { version = "0.4.21", optional = true }
clap = "2.23.0"
counted-array = "0.1"
directories = "4.0.1"
