updated osm-bright.imposm.mml so that make.py doesn't crash w/ imposm importer

[ghost]

Proposed change fixes the following error.

Traceback (most recent call last):
File "make.py", line 128, in
build()
File "make.py", line 41, in build
template = loads(templatefile.read())
File "C:\Python27\lib\json__init__.py", line 339, in loads
return _default_decoder.decode(s)
File "C:\Python27\lib\json\decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "C:\Python27\lib\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 comma on line 553 of osm-bright.imposm.mml prevents line 41 in make.py from parsing the file correctly. Affects users running make.py with config["importer"] = "imposm" in configure.py.

[hmeine]

This fixes an obvious json syntax error and tackles #105. Could this be merged please?