Update http_req requirement from 0.5 to 0.6

Updates the requirements on [http_req](https://github.com/jayjamesjay/http_req) to permit the latest version.
- [Release notes](https://github.com/jayjamesjay/http_req/releases)
- [Commits](https://github.com/jayjamesjay/http_req/compare/v0.5.0...v0.6.0)

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

acmed/Cargo.toml

@@ -16,7 +16,7 @@ publish = false
 acme_common = { path = "../acme_common" }
 clap = "2.32"
 handlebars = "3.0"
-http_req = "0.5"
+http_req = "0.6"
 log = "0.4"
 nom = "5.0"
 openssl-sys = "0.9"