pointing to main module in package.json

When I trying to use this module under node 0.10.x

```js
    var tco = require('tail-call').recur;
```
I got error:
```
Error: Cannot find module 'tail-call'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
```

Pointing up the main file in the package.json resolves this issue.