Bugfix, fix contract name conflicts in oz and mocks

Author: guo-yu

contracts/mocks/Multicall.sol

@@ -9,7 +9,7 @@ pragma experimental ABIEncoderV2;
 /// @author Joshua Levine <joshua@makerdao.com>
 /// @author Nick Johnson <arachnid@notdot.net>
 
-contract Multicall {
+contract MulticallV1 {
     struct Call {
         address target;
         bytes callData;

deploy/mocks.js

@@ -8,7 +8,7 @@ module.exports = async ({ getNamedAccounts, deployments }) => {
   };
 
   await deploy('CallReceiverMock', deployConfig);
-  await deploy('Multicall', deployConfig);
+  await deploy('MulticallV1', deployConfig);
 };
 
 module.exports.tags = ['Mocks'];