reactivate module partner_prepayment
port models and methods to api 8.0
add summary
fix yaml tests
remove FIXME, add comment

It is probably a problem with the yaml import, so no need to FIXME here.
Merge pull request #39 from yvaucher/8.0-partner_prepayment

8.0 port module partner_prepayment
scaffold new addon
partial implementation

wizard view is there, more work needed to customize the procurment handling
manage sourcing on the procurements
added pot file
split sale_order.py in smaller bits, some flake8 cleanup
rename models/ to model/
use @api.model decorator where needed

+ flake8 clean of sale_order
remove unused demo.xml file
updated travis config

remove travis_run_flake8
set a separate env for testing sale_quotation_sourcing, and it seems that
another addon is changing a return value which makes our tests unhappy