Log Message:
Updated cad/py-gds to 1.2.1

### Version 1.2.1 (Dec 5, 2017)
* `GdsLibrary` can be created directly from a GDSII file
* Added return value to `GdsLibrary.read_gds`
* Fix return value of `GdsLibrary.add`

### Version 1.2 (Oct 21, 2017)
* Added new `gdsii_hash` function.
* Added `precision` parameter to `_chop`, `Polygon.fracture`,
  `Polygon.fillet`, `PolygonSet.fracture`, `PolygonSet.fillet`, and
  `slice`.
* Included labels in flatten operations (added `get_labels` to `Cell`,
  `CellReference`, and `CellArray`).
* Fixed bug in the bounding box cache of reference copies.
* Fixed bug in `_chop` that affected `Polygon.fracture`,
  `PolygonSet.fracture`, and `slice`.
* Other minor bug fixes.