Display the environment flag in the activate command for non-default environments.

Regardless of what environment you `deploy:index` to, you'll see an activate command that assumes the default environment.

```
$ ember deploy:index --environment=production
version: 0.1.2
Connected to Redis [greeneye.redistogo.com:9585]

Successfully deployed to Redis: dist/index.html

To activate index.html:
ember activate 8f057b1519
```

If I just blindly run this command it fails.

```
$ ember activate 8f057b1519
version: 0.1.2
The `ember activate` command requires index config to be set.  For more details see http://github.com/achambers/ember-cli-deploy
```

There's room for improvement in the error message, but it's telling you that `config/deploy/development.js` doesn't exist.

My changes add the `--environment` flag to the activate command when not running in the default environment.

```
$ ember deploy:index --environment=production
version: 0.1.2
Connected to Redis [greeneye.redistogo.com:9585]

Successfully deployed to Redis: dist/index.html

To activate index.html:
ember activate 8f057b1519 --environment=production
```