qdl-xau-contango.ipynb: Solve #2 by v5 & p6.16.0428 upgrades

Switch from fecon to fecon235 for main import module.
Minor edits given more data and change in futures basis.

During 2015 we detected strong negative correlation between price change and
tango, however, in 2016 that strong correlation became positive -- thus we
conclude the relationship is spurious. The observed correlations are mere
artifacts which do not imply any significant economic relationships.