convert SignatureBouncer to initializers

contracts/drafts/SignatureBouncer.sol

@@ -1,5 +1,6 @@
 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "../access/roles/SignerRole.sol";
 import "../cryptography/ECDSA.sol";
 
@@ -28,7 +29,7 @@ import "../cryptography/ECDSA.sol";
  * Also non fixed sized parameters make constructing the data in the signature
  * much more complex. See https://ethereum.stackexchange.com/a/50616 for more details.
  */
-contract SignatureBouncer is SignerRole {
+contract SignatureBouncer is Initializable, SignerRole {
   using ECDSA for bytes32;
 
   // Function selectors are 4 bytes long, as documented in
@@ -64,6 +65,10 @@ contract SignatureBouncer is SignerRole {
     _;
   }
 
+  function initialize() public initializer {
+    SignerRole.initialize();
+  }
+
   /**
    * @dev is the signature of `this + sender` from a signer?
    * @return bool

contracts/mocks/SignatureBouncerMock.sol

@@ -1,10 +1,15 @@
 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "../drafts/SignatureBouncer.sol";
 import "./SignerRoleMock.sol";
 
 
-contract SignatureBouncerMock is SignatureBouncer, SignerRoleMock {
+contract SignatureBouncerMock is Initializable, SignatureBouncer, SignerRoleMock {
+  constructor() public {
+    SignatureBouncer.initialize();
+  }
+
   function checkValidSignature(address account, bytes signature)
     public
     view