Malformed json in imposm.mml

Hi,

In `osm-bright/osm-bright.imposm.mml` there's a trailing comma on line 553.
```json
....
552: "geometry_field": "geometry",
553: "extent": "-20037508.34 -20037508.34 20037508.34 20037508.34",
554: },
...
```

And ./make.py fails with:
```
Traceback (most recent call last):
File "./make.py", line 128, in
build()
File "./make.py", line 41, in build
template = loads(templatefile.read())
File "/usr/lib/python2.7/json/__init__.py", line 339, in loads
return _default_decoder.decode(s)
File "/usr/lib/python2.7/json/decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python2.7/json/decoder.py", line 380, in raw_decode
obj, end = self.scan_once(s, idx)
ValueError: Expecting property name: line 554 column 7 (char 26688)
The command 'python2 ./make.py' returned a non-zero code: 1
```

Inspect osm-bright/osm-bright.imposm.mml around lines 553-554, then run ./make.py or python2 ./make.py to reproduce the JSON parsing failure. Done means the template is valid JSON and the build command completes without this error.

Written by the indexing model from the issue text.