3.0 - Updating blog's spanish translations

I'm currently working on this, so, please, do not merge it!

This PR is just to inform that this section is being translated.
Esta parte a cambiado en la versión inglesa porque la manera más sencilla de ponerse en marcha es de utilisar `Composer`.
http://book.cakephp.org/3.0/en/getting-started.html#getting-cakephp