don't require execjs until needed inside the task
don't require execjs until needed inside the task

test plan: rake canvas:compile_assets should still succeed

Change-Id: Id90e7cdc9e9989bc975a09fb34c944a611074aa0
Reviewed-on: https://gerrit.instructure.com/34309
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Mark Severson <cdf1f90aee9d0d073a2a24141ebc71887f3ba533@instructure.com>
Product-Review: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
QA-Review: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

utkarshx/canvas-lms,efrj/canvas-lms,SwinburneOnline/canvas-lms,samuels410/greatlakes-lms,instructure/canvas-lms,wrdsb/canvas-lms,kyroskoh/canvas-lms,AndranikMarkosyan/lms,efrj/canvas-lms,antoniuslin/canvas-lms,juoni/canvas-lms,Rvor/canvas-lms,fronteerio/canvas-lms,grahamb/canvas-lms,AndranikMarkosyan/lms,tgroshon/canvas-lms,shidao-fm/canvas-lms,redconfetti/canvas-lms,tgroshon/canvas-lms,sdeleon28/canvas-lms,Jyaasa/canvas-lms,greyhwndz/canvas-lms,va7map/canvas-lms,ShreniiNepal/shrenii_lmp,tgroshon/canvas-lms,utkarshx/canvas-lms,wye0220/shindig-canvas,gbleydon/canvas-lms,narigone/canvas,fronteerio/canvas-lms,roxolan/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,ShreniiNepal/shrenii_lmp,instructure/canvas-lms,wrdsb/canvas-lms,skmezanul/canvas-lms,snehakachroo1/LSI_replica,instructure/canvas-lms,efrj/canvas-lms,Rvor/canvas-lms,Jyaasa/canvas-lms,Rvor/canvas-lms,narigone/canvas,roxolan/canvas-lms,samuels410/greatlakes-lms,tgroshon/canvas-lms,juoni/canvas-lms,efrj/canvas-lms,sfu/canvas-lms,jay3126/canvas-lms,utkarshx/canvas-lms,shidao-fm/canvas-lms,roxolan/canvas-lms,eCNAP/CANVAS,Unow/canvas-lms,Jyaasa/canvas-lms,greyhwndz/canvas-lms,ShreniiNepal/shrenii_lmp,antoniuslin/canvas-lms,sdeleon28/canvas-lms,leftplusrightllc/canvas-lms,sdeleon28/canvas-lms,SwinburneOnline/canvas-lms,kyroskoh/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,wrdsb/canvas-lms,Unow/canvas-lms,AndranikMarkosyan/lms,leftplusrightllc/canvas-lms,usmschuck/canvas,sfu/canvas-lms,sdeleon28/canvas-lms,jay3126/canvas-lms,narigone/canvas,va7map/canvas-lms,jay3126/canvas-lms,Unow/canvas-lms,matematikk-mooc/canvas-lms,jay3126/canvas-lms,ShreniiNepal/shrenii_lmp,roxolan/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,greyhwndz/canvas-lms,eCNAP/CANVAS,venturehive/canvas-lms,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,snehakachroo1/LSI_replica,wye0220/shindig-canvas,skmezanul/canvas-lms,gbleydon/canvas-lms,shidao-fm/canvas-lms,sfu/canvas-lms,AndranikMarkosyan/lms,whalejasmine/canvas-lms,juoni/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,antoniuslin/canvas-lms,HotChalk/canvas-lms,utkarshx/canvas-lms,samuels410/greatlakes-lms,dgynn/canvas-lms,fronteerio/canvas-lms,djbender/canvas-lms,va7map/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,greyhwndz/canvas-lms,antoniuslin/canvas-lms,matematikk-mooc/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,juoni/canvas-lms,leftplusrightllc/canvas-lms,wrdsb/canvas-lms,skmezanul/canvas-lms,skmezanul/canvas-lms,venturehive/canvas-lms,redconfetti/canvas-lms,leftplusrightllc/canvas-lms,whalejasmine/canvas-lms,wye0220/shindig-canvas,redconfetti/canvas-lms,whalejasmine/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,va7map/canvas-lms,shidao-fm/canvas-lms,samuels410/greatlakes-lms,Rvor/canvas-lms,Jyaasa/canvas-lms,dgynn/canvas-lms,kyroskoh/canvas-lms,snehakachroo1/LSI_replica,dgynn/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,eCNAP/CANVAS,sfu/canvas-lms,matematikk-mooc/canvas-lms,whalejasmine/canvas-lms,narigone/canvas,kyroskoh/canvas-lms,gbleydon/canvas-lms,SwinburneOnline/canvas-lms,snehakachroo1/LSI_replica,sfu/canvas-lms,venturehive/canvas-lms,usmschuck/canvas,eCNAP/CANVAS,usmschuck/canvas,usmschuck/canvas,djbender/canvas-lms