chore: use abi encoder v2 on our main contracts to be able to work with structs (#166)

Author: abarmat

contracts/DisputeManager.sol

@@ -1,4 +1,5 @@
 pragma solidity ^0.6.4;
+pragma experimental ABIEncoderV2;
 
 /*
  * @title Dispute management

contracts/GNS.sol

@@ -1,4 +1,5 @@
 pragma solidity ^0.6.4;
+pragma experimental ABIEncoderV2;
 
 import "./Governed.sol";
 

contracts/GraphToken.sol

@@ -1,4 +1,5 @@
 pragma solidity ^0.6.4;
+pragma experimental ABIEncoderV2;
 
 /*
  * @title GraphToken contract

contracts/RewardsManager.sol

@@ -1,4 +1,5 @@
 pragma solidity ^0.6.4;
+pragma experimental ABIEncoderV2;
 
 import "./Governed.sol";
 

contracts/ServiceRegistry.sol

@@ -1,4 +1,5 @@
 pragma solidity ^0.6.4;
+pragma experimental ABIEncoderV2;
 
 import "./Governed.sol";
 

contracts/Staking.sol

@@ -1,4 +1,5 @@
 pragma solidity ^0.6.4;
+pragma experimental ABIEncoderV2;
 
 /*
  * @title Staking contract