Bug 29320: Update OverDrive availability to V2

Bug 29320: Update OverDrive availability to V2

This patch updates the call to overdrive_proxy to allow passing a version
and updates the code to use V2 for availability

To test:
1 - Enable OverDrive integration by setting the sysprefs
2 - Log in to your overdrive account in the Koha opac
3 - Perform a search and note availability display
4 - Confirm you can checkout/hold/return books
5 - Apply patch
6 - Repeat search and test functionality
7 - Note that title with '0 out of 0 copies available' now say 'Currently unavailable'

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>