Relax minimal patch version for serde_derive

A conflict with cbindgen exists which prevent restson to be used
with it. `cbindgen` [pins `serde_derive` to `1.0.58`](https://github.com/eqrion/cbindgen/blob/44e9b2112a06e46ddda6073e237d3a56df39c3e2/Cargo.toml#L24-L27)
due to mozilla/cbindgen#203
so depending on `^1.0.80` prevents using both crates together.

This patch simply relaxes the dependency on `serde_derive` to `1.0`,
which is API compatible with both `1.0.80` and `1.0.58`.

Closes #14

Cargo.toml

@@ -16,7 +16,7 @@ futures = "^0.1.25"
 tokio-core = "^0.1.17"
 serde = "^1.0.80"
 serde_json = "^1.0.33"
-serde_derive = "^1.0.80"
+serde_derive = "1.0"
 url = "^1.7.2"
 log = "^0.4.6"
 base64 = "^0.10.0"