Fix check for string packageFiles config (#307)

lib/index.js

@@ -150,7 +150,10 @@ async function findPackageFiles(config) {
 
 async function getAllRepoUpgrades(repo) {
   let upgrades = [];
-  for (const packageFile of repo.packageFiles) {
+  for (let packageFile of repo.packageFiles) {
+    if (typeof packageFile === 'string') {
+      packageFile = { fileName: packageFile };
+    }
     const cascadedConfig = configParser.getCascadedConfig(repo, packageFile);
     upgrades = upgrades.concat(
       await worker.processPackageFile(