Gulpfile: Remove let hack

Let hack is not necessary as the test is being passed without it and it makes var in blocks fail.
To get rid of 'let unexpected token' we must either configure traceur to set blockBindings to true or just get rid of the hack and use both var and let properly.
This also fixes the fake build failure from previous commit f0d33f64730fbd194a04ea4c662ebe5c964e9338