make sure webpack output dir exists
make sure webpack output dir exists

Change-Id: I18b0e9873c7921e37f9253433533f44ec13983da
Reviewed-on: https://gerrit.instructure.com/105231
Reviewed-by: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
Product-Review: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
QA-Review: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
Tested-by: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>

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