Bump structopt from 0.3.14 to 0.3.15 (paritytech#148)

Bumps [structopt](https://github.com/TeXitoi/structopt) from 0.3.14 to 0.3.15.
- [Release notes](https://github.com/TeXitoi/structopt/releases)
- [Changelog](https://github.com/TeXitoi/structopt/blob/master/CHANGELOG.md)
- [Commits](TeXitoi/structopt@v0.3.14...v0.3.15)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

bridges/bin/node/node/Cargo.toml

@@ -14,7 +14,7 @@ name = "bridge-node"
 futures = "0.3.5"
 jsonrpc-core = "14.2.0"
 log = "0.4.8"
-structopt = "0.3.14"
+structopt = "0.3.15"
 bridge-node-runtime = { version = "0.1.0", path = "../runtime" }
 sp-bridge-eth-poa = { version = "0.1.0", path = "../../../primitives/ethereum-poa" }