Fix double and typo in date preposition

The date of occurrence preposition in the schema contains an 'and',
which is not needed as it always comes last and is formed with
to_sentence, adding the 'and' for us.
Merge pull request #43 from alphagov/fix-double-and-typo

Fix double and typo in date preposition
Make date_of_occurrence make sense

Previously, the date_of_occurrence preposition didn't make sense. This
commit fixes that.
Merge pull request #44 from alphagov/fix-date-preposition

Make date_of_occurrence make sense