clodius aggregate: NameError: global name '_bigwig' is not defined

On ubuntu xenial with a fresh python 2.7.12 install, I get the following:

```bash
root@609c5b8e4115:/$ clodius aggregate bigwig HistonH3_cre_trt_rep1.bw
/usr/local/lib/python2.7/dist-packages/h5py/__init__.py:36: FutureWarning: Conversion of the second argument of issubdtype from `float` to `np.floating` is deprecated. In future, it will be treated as `np.float64 == np.dtype(float).type`.
from ._conv import register_converters as _register_converters
Traceback (most recent call last):
File "/usr/local/bin/clodius", line 11, in
sys.exit(cli())
File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 722, in __call__
return self.main(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/clodius/cli/aggregate.py", line 1347, in bigwig
_bigwig(
NameError: global name '_bigwig' is not defined
```