Fix incorrect dependency

toml version `0.5.0` incorrectly specified serde version `1.0.0` as one
of its dependencies, even though it does not work with with serde
version `1.0.0`. This [was
fixed](toml-rs/toml-rs#311) and released in
version `0.5.2`.

This commit updates the toml dependency to version `0.5.2`. Now
`proc-macro-crate` builds fine with the cargo flag
[`-Z minimal-versions`](rust-lang/cargo#5657).

Cargo.toml

@@ -14,7 +14,7 @@ Replacement for crate (macro_rules keyword) in proc-macros
 readme = "./README.md"
 
 [dependencies]
-toml = "0.5.0"
+toml = "0.5.2"
 
 [dev-dependencies]
 quote = "1.0.7"