Don't allow null stockrecords in basket lines

This finishes the changes made in v0.6 where basket lines started
requiring a stockrecord FK. v0.7 handles the upgrade in middleware by
updating basket lines that didn't have a stockrecord. But since the
patch for v0.7 requires additional database queries, it's time to remove
it - which is what this change does.

This will be a slightly fiddly upgrade for some people as they will need
to run a data migration on their DB before applying the new basket
migration. I've included a sample snippet in the release notes which
should help.
Use a default of '' for basket line stockrecord  migration

This is a bit of stab in the dark. I saw the travis tests were failing
as we're not allowed to use a default of 0 when altering the schema.j
Correct how env variables are accessed in makefile
Document the migration testing process better
Update wording of release notes regarding the field changes

I just wanted to change one word, and then got sidetracked with
reordering the sentences.