refactor: replace deprecated String.prototype.substr() (#291)

Author: CommanderRoot

src/index.js

@@ -265,10 +265,7 @@ class CompressionPlugin {
               const index = filenameForRelatedName.indexOf("?");
 
               if (index >= 0) {
-                filenameForRelatedName = filenameForRelatedName.substr(
-                  0,
-                  index
-                );
+                filenameForRelatedName = filenameForRelatedName.slice(0, index);
               }
 
               relatedName = `${path

test/helpers/readAsset.js

@@ -10,7 +10,7 @@ export default (asset, compiler, stats) => {
   const queryStringIdx = targetFile.indexOf("?");
 
   if (queryStringIdx >= 0) {
-    targetFile = targetFile.substr(0, queryStringIdx);
+    targetFile = targetFile.slice(0, queryStringIdx);
   }
 
   try {