Fallback to Date.strptime to parse dates

Use `Date.strptime(date_string, 'mm/dd/yyyy')` as a fallback to parse dates if no other methods work. The gem would have done this in the past, but as of 1.9 Date.parse doesn't do this anymore, so calling strptime directly is necessary.

NOTE: this has been approved and merged by the maintainer of Workbook already