Fix warning for undefined array key

1.3.17

Tagging version 1.3.17

Fixed contents of "Last Update" metabox

See #21

Added custom update schedule for Product Sets

• Added Custom Update Schedule for Product Sets.
• Formatted require_once() calls differently.
• Added dfrps_get_default_update_time() function.
• Update parameter requirements for dfrps_get_next_update_time()

Fixed spelling and whitespace

Full Changelog: 1.3.12...1.3.14