Do not hard-code useragent version

[0xazure]

Currently, we hard-code the UserAgent header with a String value containing supernova's version triplet here:

supernova/src/lib.rs

Line 48 in 66f5bc5

headers.set(UserAgent::new("supernova/0.1.0"));

This should instead be generated at compile-time based on data from Cargo.toml using the env variables cargo exports.