Move bluebird from devDep to regular dep

Hi,

I ran into some issues when using this library inside another project, [sprout-static-cms](https://github.com/carrot/sprout-static-cms). This project is a template that is then consumed by [sprout](https://github.com/carrot/sprout), a CLI tool for generating projects from templates. Sprout `require`s this [init.coffee](https://github.com/carrot/sprout-static-cms/blob/master/init.coffee) file to allow interactive setup through the CLI as well as providing some hooks each template can tap into to run custom code. I'm trying to integrate `contentful-management` into one of these hooks in order to do some account setup for the user.

Currently when I'm requiring in the init file, I get the following error stack:

```
Error: Cannot find module 'bluebird'
  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)
  at Object.<anonymous> (/Users/josh/code/sprout-static-cms/node_modules/contentful-management/rate-limit.js:7:16)
```

which appears to come from [this line](https://github.com/contentful/contentful-management.js/blob/872298611a1fbd71de6ae982fdc1283f50c897e7/rate-limit.js#L7) in the code. On further inspection, it appears that `bluebird` isn't being installed when using npm because it is listed under `devDependencies` instead of a regular `dependencies`. When I make this fix, `contentful-management` works perfectly with `sprout`.