Pyxform updates

[roger.h...@kimetrica.com]

We need to be able to use `select_one_from_file` in an XLSForm which we upload to KoboToolbox and deploy for data collection using Enketo.

Currently, forms like this can't be deployed because of https://github.com/XLSForm/pyxform/issues/87

I'd like to fix that (I can confirm that Martijn's suggestion works) and submit a PR, but I am not sure which branch to base the required changes against.

https://github.com/XLSForm/pyxform seems to be the master repo, and it includes https://github.com/XLSForm/pyxform/commit/fab7fce8d22163a1c017c07d2c58b149050b96ca which closes https://github.com/XLSForm/pyxform/issues/30 and provides the `select_one_from_file` support.

However, https://github.com/kobotoolbox/pyxform seems to have diverged significantly from https://github.com/XLSForm/pyxform but seems to be the one that Kobocat is using (`requirements/base.pip` has `git+https://github.com/kobotoolbox/pyxform.git#egg=pyxform-dev`). That branch doesn't seem to have https://github.com/XLSForm/pyxform/commit/fab7fce8d22163a1c017c07d2c58b149050b96ca.

I don't think I can easily switch back to Master or to https://github.com/kobotoolbox/pyxform/tree/xlsform-master because the KoboToolbox master includes commits that add functionality, e.g. the SPSS Labels, that isn't in the XLSForm/pyxform.

Do you have any recommendations on the best way to get the `select_one_from_file` functionality into the version of Pyxform that is used by Kobocat?

Thanks

Roger