refactor: Replace reqwest library with ureq

This replaces reqwest with the more simplistic ureq library for
performing required HTTP requests.

Reqwest comes with a lot of (tokio-based) machinery for
high-performance requesting that is a bit out of scope for
journaldriver's needs.

This clocks in at 62 (!) fewer dependencies after the change, with
equivalent functionality. Wew.

Cargo.toml

@@ -8,15 +8,14 @@ license = "GPL-3.0-or-later"
 chrono = { version = "0.4", features = [ "serde" ]}
 env_logger = "0.5"
 failure = "0.1"
-hyper = "0.11"
 lazy_static = "1.0"
 log = "0.4"
 medallion = "2.2"
-reqwest = "0.8"
 serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"
 systemd = "0.3"
+ureq = { version = "0.6.2", features = [ "json" ]}
 
 [build-dependencies]
 pkg-config = "0.3"