Bump structopt from 0.3.17 to 0.3.18 (#713)

Bumps [structopt](https://github.com/TeXitoi/structopt) from 0.3.17 to 0.3.18.
- [Release notes](https://github.com/TeXitoi/structopt/releases)
- [Changelog](https://github.com/TeXitoi/structopt/blob/master/CHANGELOG.md)
- [Commits](TeXitoi/structopt@v0.3.17...v0.3.18)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

boa_cli/Cargo.toml

@@ -14,7 +14,7 @@ edition = "2018"
 Boa = { path = "../boa", features = ["serde"] }
 rustyline = "6.3.0"
 rustyline-derive = "0.3.1"
-structopt = "0.3.17"
+structopt = "0.3.18"
 serde_json = "1.0.57"
 colored = "2.0.0"
 regex = "1.3.9"

tester/Cargo.toml

@@ -12,7 +12,7 @@ edition = "2018"
 
 [dependencies]
 Boa = { path = "../boa" }
-structopt = "0.3.17"
+structopt = "0.3.18"
 serde = { version = "1.0.116", features = ["derive"] }
 serde_yaml = "0.8.13"
 serde_json = "1.0.57"