remove dependency on runtime-fmt (#773)

Per the issue, "runtime-fmt uses perma-unstable rust APIs and is
therefore susceptible to breakage".

This replaces the two calls to rt_format! with .replace() and drops the
dependency.

Fixes #769

src/main.rs

@@ -6,8 +6,6 @@ extern crate gettext_macros;
 #[macro_use]
 extern crate rocket;
 #[macro_use]
-extern crate runtime_fmt;
-#[macro_use]
 extern crate serde_json;
 #[macro_use]
 extern crate validator_derive;