Fix chdir to npm package name (should be to repo name)

npm-clone does chdir to the package name, though there are packages whose repo name and package name are different. For example, when I do:

```
$ npm-clone octokit
Cloning into 'octokit.js'...

/usr/local/lib/node_modules/npm-clone/node_modules/repo-url/node_modules/silent-npm-registry-client/node_modules/npm-registry-client/node_modules/graceful-fs/polyfills.js:14
  chdir.call(process, d)
        ^
Error: ENOENT, no such file or directory
    at process.chdir (/usr/local/lib/node_modules/npm-clone/node_modules/repo-url/node_modules/silent-npm-registry-client/node_modules/npm-registry-client/node_modules/graceful-fs/polyfills.js:14:9)
    at ChildProcess.<anonymous> (/usr/local/lib/node_modules/npm-clone/index.js:25:13)
    at ChildProcess.EventEmitter.emit (events.js:98:17)
    at Process.ChildProcess._handle.onexit (child_process.js:789:12)
```

npm package name is ```octokit```, while the repo name is ```octokit.js```.