Remove payable fallback from IStakingProxy, fix linting errors

contracts/staking/contracts/src/StakingProxy.sol

@@ -92,6 +92,7 @@ contract StakingProxy is
         external
         onlyAuthorized
     {
+        // solhint-disable-next-line not-rely-on-time
         uint96 timestamp = block.timestamp.downcastToUint96();
         if (shouldSetReadOnlyMode) {
             stakingContract = readOnlyProxy;

contracts/staking/contracts/src/ZrxVaultBackstop.sol

@@ -36,7 +36,7 @@ contract ZrxVaultBackstop {
     /// @param _stakingProxyAddress Address of stakingProxy.
     /// @param _zrxVaultAddress Address of zrxVault.
     constructor(
-        address payable _stakingProxyAddress,
+        address _stakingProxyAddress,
         address _zrxVaultAddress
     )
         public
@@ -59,7 +59,9 @@ contract ZrxVaultBackstop {
         );
 
         // Ensure that the stakingProxy has been in read-only mode for a long enough time
+        // TODO: Ensure correct value is set in production
         require(
+            // solhint-disable-next-line not-rely-on-time
             block.timestamp.safeSub(readOnlyState.lastSetTimestamp) >= 40 days,
             "READ_ONLY_MODE_DURATION_TOO_SHORT"
         );

contracts/staking/contracts/src/interfaces/IStakingProxy.sol

@@ -22,8 +22,7 @@ pragma experimental ABIEncoderV2;
 import "./IStructs.sol";
 
 
-contract IStakingProxy /* is IStaking */
-{
+contract IStakingProxy {
 
     /// @dev Emitted by StakingProxy when a staking contract is attached.
     /// @param newStakingContractAddress Address of newly attached staking contract.
@@ -40,12 +39,6 @@ contract IStakingProxy /* is IStaking */
         uint96 timestamp
     );
 
-    /// @dev Delegates calls to the staking contract, if it is set.
-    // solhint-disable no-complex-fallback
-    function ()
-        external
-        payable;
-
     /// @dev Attach a staking contract; future calls will be delegated to the staking contract.
     /// Note that this is callable only by an authorized address.
     /// @param _stakingContract Address of staking contract.