refactor: replace deprecated String.prototype.substr() (#950)

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

lib/copy/__tests__/ncp/ncp.test.js

@@ -33,7 +33,7 @@ describe('ncp', () => {
 
     describe('when copying files using filter', () => {
       before(cb => {
-        const filter = name => name.substr(name.length - 1) !== 'a'
+        const filter = name => name.slice(-1) !== 'a'
 
         rimraf(out, () => ncp(src, out, { filter }, cb))
       })
@@ -45,7 +45,7 @@ describe('ncp', () => {
               const curFile = files[fileName]
               if (curFile instanceof Object) {
                 filter(curFile)
-              } else if (fileName.substr(fileName.length - 1) === 'a') {
+              } else if (fileName.slice(-1) === 'a') {
                 delete files[fileName]
               }
             }