Update docs to use npx to install locally

[giocodes]

Hi! With the latest node and NPM version I believe that we can do:
npx commitizen init cz-conventional-changelog --save-dev --save-exact

I'll be happy to create a PR if this is something we want to update the docs.

[jimthedev]

Yes!!! That’d be great. We need node 8 9 and 10 added to our Travis yaml too.

[mtliendo]

Is this still open to be picked up?

[jimthedev]

Yeah i think it is open

[giocodes]

Sorry, I intended to do this but I got lost on the Travis yaml comment. Is that also on the documentation? @jimthedev

[LinusU]

That doesn't have to be the same PR, feel free to just address the updated documentation 👍

[LinusU]

addressed the Travis thing here: #531

[giocodes]

Cool, I'll create a PR soon for the documentation changes. Thanks