Fixed relative imports. (#1321)

contracts/mocks/InsecureInvariantTargetBounty.sol

@@ -3,7 +3,7 @@ pragma solidity ^0.4.24;
 // When this line is split, truffle parsing fails.
 // See: https://github.com/ethereum/solidity/issues/4871
 // solium-disable-next-line max-len
-import {BreakInvariantBounty, Target} from "../../contracts/bounties/BreakInvariantBounty.sol";
+import {BreakInvariantBounty, Target} from "../bounties/BreakInvariantBounty.sol";
 
 
 contract InsecureInvariantTargetMock is Target {

contracts/mocks/SecureInvariantTargetBounty.sol

@@ -3,7 +3,7 @@ pragma solidity ^0.4.24;
 // When this line is split, truffle parsing fails.
 // See: https://github.com/ethereum/solidity/issues/4871
 // solium-disable-next-line max-len
-import {BreakInvariantBounty, Target} from "../../contracts/bounties/BreakInvariantBounty.sol";
+import {BreakInvariantBounty, Target} from "../bounties/BreakInvariantBounty.sol";
 
 
 contract SecureInvariantTargetMock is Target {