Add optimism-kovan to EtherscanProvider (#3135).
Add optimism-kovan to EtherscanProvider (#3135).

ethers-io/ethers.js,ethers-io/ethers.js,ethers-io/ethers.js,ethers-io/ethers.js,ethers-io/ethers.js,ethers-io/ethers.js,ethers-io/ethers.js