fix aspect ratio in unsplash image search
fix aspect ratio in unsplash image search

test plan:
 - set up unsplash plugin
 - do a course card image search for "cat"
 - notice the vertically-cropped images aren't stretched
 - do a course card image search for "panorama"
 - notice the horizontally-cropped images aren't squished

fixes ADMIN-2667

Change-Id: I068cdfce055ff2641de7a0db974bdb977c55a225
Reviewed-on: https://gerrit.instructure.com/193792
Tested-by: Jenkins
Reviewed-by: Carl Kibler <7c2b40129dd885a147d5b2dda56dda016b9a8f57@instructure.com>
QA-Review: Carl Kibler <7c2b40129dd885a147d5b2dda56dda016b9a8f57@instructure.com>
Product-Review: Laura Leavitt <5006d6b46871eacefaf5eef3c8e9c93df19e43c2@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms