Update toml requirement from 0.5.6 to 0.8.8

Updates the requirements on [toml](https://github.com/toml-rs/toml) to permit the latest version.
- [Commits](toml-rs/toml@toml-v0.5.10...toml-v0.8.8)

---
updated-dependencies:
- dependency-name: toml
  dependency-type: direct:production
...

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

structopt-toml/Cargo.toml

@@ -21,7 +21,7 @@ default = ["clap/default", "structopt/default"]
 [dependencies]
 clap                  = { version = "2.33.0", default-features = false }
 anyhow                = "1.0.42"
-toml                  = "0.5.6"
+toml                  = "0.8.8"
 serde                 = "1.0.104"
 serde_derive          = "1.0.104"
 structopt             = { version = "0.3.11", default-features = false }