refactor contrats with aragonos

apps/contribution/contracts/misc/Migrations.sol

@@ -0,0 +1,23 @@
+pragma solidity ^0.4.4;
+
+contract Migrations {
+  address public owner;
+  uint public last_completed_migration;
+
+  modifier restricted() {
+    if (msg.sender == owner) _;
+  }
+
+  function Migrations() {
+    owner = msg.sender;
+  }
+
+  function setCompleted(uint completed) restricted {
+    last_completed_migration = completed;
+  }
+
+  function upgrade(address new_address) restricted {
+    Migrations upgraded = Migrations(new_address);
+    upgraded.setCompleted(last_completed_migration);
+  }
+}