Don't modify the translation store

This was causing this test (https://github.com/alphagov/whitehall/blob/e93bca0eb16b2a1c38309033d62bb0408c345680/test/unit/helpers/document_helper_test.rb#L198)
to sporadically fail because the translation for "Minister" was being
overwritten in the translation store and so it was being translated as
'minister-value'.  I did look into adding a teardown that reset the
translation store, but this is very slow (blowing away and reloading
all translations).

Although there is now the risk that these tests
will break if the French translations for these words should ever
change, I think this is better than stubbing the translation helper,
which feels way too much like testing implementation.
Merge pull request #863 from alphagov/fix-parallel-test-failure

Don't modify the translation store