Bug 20737: Use https for baker and taylor cover images

Bug 20737: Use https for baker and taylor cover images

Easy change, should be able to verify with code review or testing with
dummy values

To test:
1 - Put some values in baker and taylor prefs (don't need to be valid)
2 - Do a search on the opac (and have some items with isbns)
3 - Inspect the cover images links to ensure they are formed correctly
4 - prove -v t/External/BakerTaylor.t

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>