Add missing tests for commit 75e49fe0

This now makes us fairly sure of what the following commit does:

  * https://github.com/aptivate/ckanext-datasetversions/commit/75e49fe08ae8946bcc1fc2614a6b66cacea824ac.

Closes https://github.com/aptivate/ckanext-datasetversions/issues/4.

We do this to maintain the standard API when calling this
function from the tests using the `_get_helper` function and
in general, with the `helpers` module.

Since the `_get_context` function is now available
in the helpers modules, we import and use that one,
to avoid having two differing implementations.

Fix failing Travis CI build

We will match Map Actions postgres version and won't have to handle
errors when postgres fails to boot up (as has been seen with lesser
versions).

This will avoid errors when pinned versions are not
discoverable from the Travis PPAs configured.

Before this fix we were losing information in the context when
reindexing and the dictization_functions validate method was
causing the product theme tags to be moved from tags in the
dataset dict to 'product_themes'. This was causing problems
when Solr would complain about 'product_themes' not being
multivalue.

With this fix, product_themes are correctly stored as tags
in the dataset dict so they get converted to vocab_* fields
in Solr (which is mutivalue) and Solr doesn't complain

Added copyright and credits.

When ignore_auth is True there may not be a user.