Remove UNIX date indexing configuration

Using UNIX date format creates problems with sorting news before 2001 (and older than 2026).
Merge pull request #6 from dimaip/patch-1

[!!!][BUGFIX] Use ISO 8601 for datetime properties

Prevents errors with older dates where unix timestamps do not work as expected.

Breaking as you need to reindex with this change as otherwise you will have mixed formats in the index.