Fixes `Cannot redefine property: promise` error.

Modules that are linked in during development end up having multiple instance of `songbird` required and they all try to define `promise`. This patch fixes that.

```
/app/node_modules/songbird/lib/songbird.js:59
    return Object.defineProperty(target, "promise", {
                  ^
TypeError: Cannot redefine property: promise
  at Function.defineProperty (native)
  at definePromiseProperty (/app/node_modules/songbird/lib/songbird.js:59:19)
  at Object.<anonymous> (/app/node_modules/songbird/lib/songbird.js:87:5)
  at Object.<anonymous> (/app/node_modules/songbird/lib/songbird.js:90:4)
  at Module._compile (module.js:456:26)
  at Object.Module._extensions..js (module.js:474:10)
...
```