Revert "Gulpfile: Remove let hack"

The `let` hack is completely and utterly intentional.

Declare variables in the scope they're used in. There is no reason to
remove the let hack unless you want bad code to pass. I do not want
bad code to pass.

This reverts commit 0b76e245b6bcffdeb343d21a2e90aebab2a8d027.