Roles.add and remove now require pre-conditions on the account. (#1421)

(cherry picked from commit 3bd30f7382)

contracts/access/Roles.sol

@@ -14,6 +14,8 @@ library Roles {
    */
   function add(Role storage role, address account) internal {
     require(account != address(0));
+    require(!has(role, account));
+
     role.bearer[account] = true;
   }
 
@@ -22,6 +24,8 @@ library Roles {
    */
   function remove(Role storage role, address account) internal {
     require(account != address(0));
+    require(has(role, account));
+
     role.bearer[account] = false;
   }