Upgrade handlebars to 4.3.6.

[darnuria]

Had an issue due to compiling error of handlebars tried to update it went smoothly. :)

error: expected `;`, found keyword `let`
   --> /home/darnuria/.cargo/registry/src/github.com-1ecc6299db9ec823/handlebars-3.5.3/src/helpers/mod.rs:180:81
    |
180 |                 write!(out, "{}:{}{}"->"", h.name(), v.value().render(), output)
    |                                                                                 ^ help: add `;` here
181 |                 let output = format!(, );
    |                 --- unexpected token

   Compiling futures-util v0.3.13
error: could not compile `handlebars` due to previous error
warning: build failed, waiting for other jobs to finish...

[darnuria]

@mardiros Hello if you want some in person time for a review will be available next week! :)