build: Bump non-qunit devDependencies to latest where possible
build: Bump non-qunit devDependencies to latest where possible

 grunt                          1.0.2  →   1.0.3
 grunt-contrib-uglify           3.3.0  →   3.4.0
 grunt-contrib-watch            1.0.0  →   1.1.0
 imagemin                       5.3.1  →   6.0.0
 prismjs                       1.12.2  →  1.15.0

Change-Id: Icfee05bbe2a7bf85ab2edb26d501f5c7772b92ca

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