Fix broken --loadmain

As pointed out in [the comment](https://github.com/toddself/repl-it/pull/7#issuecomment-58570792), there is a context issue that makes `--loadmain` crash.
```
$ repl-it --loadmain

repl-it/index.js:34
          if(this.opts.verbose){
                 ^
TypeError: Cannot read property 'opts' of undefined
    at loadMain (repl-it/index.js:34:18)
    at null.<anonymous> (repl-it/index.js:42:11)
    at Replit.loadPackages (repl-it/index.js:126:6)
    at repl-it/index.js:24:20
    at repl-it/index.js:95:8
    at fs.js:271:14
    at Object.oncomplete (fs.js:107:15)
```