Update the quick start on the main page

[Javier Alfonso]

In the web page, the commands for a quick start are:

$ composer require florianv/swap php-http/message php-http/guzzle6-adapter

But in the GitHub is:

$ composer require php-http/curl-client nyholm/psr7 php-http/message florianv/swap

which has the dependency nyholm/psr7.

I think the main web page should be updated.

[Florian Voutzinos]

@PhoneixS Done, thanks!