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 <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>