Add pure to tokenFallback function (#1122)

Silences warning

contracts/ownership/HasNoTokens.sol

@@ -18,7 +18,7 @@ contract HasNoTokens is CanReclaimToken {
   * @param value_ uint256 the amount of the specified token
   * @param data_ Bytes The data passed from the caller.
   */
-  function tokenFallback(address from_, uint256 value_, bytes data_) external {
+  function tokenFallback(address from_, uint256 value_, bytes data_) external pure {
     from_;
     value_;
     data_;