changes var to let in variables - #246

Okay, this one will be interesting, so since this is variables exercise itself.
Earlier we had just var, now we have let and const, we should update this exercise to educate how they work and how they are different.
We can have this change in this PR itself or another one as you like @a0viedo :)

Sounds well, we use this workshopper every meetup of nodeschool/spb community and can teach es6 tasks to our students
@AnshulMalik @a0viedo thanks for javascripting 👍

Yeah, it's a nice place to learn. You can add those changes here or in another PR @alexey-pd

if someone else would like to review the PR that would be amazing @workshopper/core