Fix: Marking a variable priced product as sold out can affect the next product in the download archive list.
Fix: purchase_link shortcode not respecting the price_id parameter.
Dev: Remove unreachable code in edd_pl_override_variable_pricing() function.
Dev: Refactor how extension gets loaded and checks for system requirements.
Dev: Refactor edd_pl_get_file_purchases() for EDD 3.0 support.
EDD 3.0: edd_pl_get_user_purchase_count() now takes into account variable prices.
