Removes transitive dep on time

As suggested by @AldaronLau in:
#100

Cargo.toml

@@ -14,7 +14,10 @@ edition = "2021"
 name = "cron"
 
 [dependencies]
-chrono = "~0.4"
+# Disabling default features eliminates a superfluous transitive
+# dependency on the `time` crate, which is susceptible to segfaults.
+# https://rustsec.org/advisories/RUSTSEC-2020-0071
+chrono = { version = "~0.4", default-features = false, features = [] }
 nom = "~7"
 once_cell = "1.5.2"