Upgrade Solidity compiler version

truffle-config.js

@@ -2,7 +2,7 @@ require('chai/register-should');
 const { GSNDevProvider } = require('@openzeppelin/gsn-provider');
 
 const solcStable = {
-  version: '0.5.7',
+  version: '0.5.11',
 };
 
 const solcNightly = {