Inactive heavens are not properly filtered

La Reunion is inactive but still shown on the angels homepage.

I think the line 

```
if 'exists' and 'active' in heaven: 
```

should be 

```
if heaven.get('exists') and heaven.get('active'):
```

Right now, the `build.py` just checks that the key in the YAML file exists and doesn't care if it's true or false.