Another small writer fix

[keflavich]

From the commit:
another fix to the writer: the open function only accepts one argument. How
did this ever work?

[coveralls]

Coverage remained the same at 63.13% when pulling bc12dca on keflavich:another_writer_fix into 1ed4673 on astropy:master.

[cdeil]

@keflavich - Thanks!
Merging this now.

I don't know if this was always broken or if a regression happened.
This is for the case of an empty shapelist, so probably very rarely executed by users.
And the whole write method as well as very large parts of pyregion/core.py are not covered by tests:
https://coveralls.io/builds/11110671/source?filename=pyregion%2Fcore.py#L160

Of course more tests would be important / welcome, see #44.