message: use exchangeRateOf from exchange contract
diff: @@ -2,6 +2,7 @@ import Web3 from 'web3';
  import { AbiItem } from 'web3-utils';
  import { Contract, ContractOptions } from 'web3-eth-contract';
  import RevenuePoolABI from '../../contracts/abi/v0.6.0/revenue-pool';
  +import ExchangeABI from '../../contracts/abi/v0.6.0/exchange';
  import ERC20ABI from '../../contracts/abi/erc-20';
  import { getAddress } from '../../contracts/addresses';
  import {
  @@ -39,11 +40,11 @@ export default class RevenuePool {
  }
  async currentTokenUSDRate(tokenAddress: string): Promise<string> {
  - let revenuePool = new this.layer2Web3.eth.Contract(
  - RevenuePoolABI as AbiItem[],
  - await getAddress('revenuePool', this.layer2Web3)
  + let exchange = new this.layer2Web3.eth.Contract(
  + ExchangeABI as AbiItem[],
  + await getAddress('exchange', this.layer2Web3)
  );
  - let { price } = await revenuePool.methods.exchangeRateOf(tokenAddress).call();
  + let { price } = await exchange.methods.exchangeRateOf(tokenAddress).call();
  return price;
  }
  