dont't use ocatal literal; typo

to prevent unexpected error in strict mode:

```
/Users/robinqu/Desktop/webspec/w3c-test-transformer/node_modules/mkdirp/index.js:19
        mode = 0777 & (~process.umask());
               ^^^^
SyntaxError: Octal literals are not allowed in strict mode.
    at exports.runInThisContext (vm.js:69:16)
    at Module._compile (module.js:432:25)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:349:32)
    at Function.Module._load (module.js:305:12)
    at Module.require (module.js:357:17)
    at require (module.js:373:17)
    at Object.<anonymous> (/Users/robinqu/Desktop/webspec/w3c-test-transformer/transform.js:9:23)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
```