build: fix flattening error

Author: abarmat

contracts/abdk-libraries-solidity/ABDKMathQuad.sol

@@ -2,7 +2,7 @@
  * ABDK Math Quad Smart Contract Library.  Copyright © 2019 by ABDK Consulting.
  * Author: Mikhail Vladimirov <[email protected]>
  */
-pragma solidity ^0.5.0 || ^0.6.0;
+pragma solidity ^0.6.4;
 
 /**
  * Smart contract library of mathematical functions operating with IEEE 754