build: Bump various devDependencies to latest
build: Bump various devDependencies to latest

 imagemin           6.0.0  →   6.1.0
 karma              3.1.3  →   3.1.4
 qunit              2.8.0  →   2.9.1

Change-Id: I5b66f9a2b820aa8d100fefa424a29d931994dc0f

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui