Change composer instructions

It just needs simple curl, no need to spend time figuring how it works, thus diverting attention. Those who want to know more can still follow link anyway.

Also make sure that after executing 'create-project' command user ends with zf2-tutorial (and not ./path/to/install), it is easier to notice what's going on that way. (I've spend some time wondering where should I get zf2-tutorial from.)

Before update change dir to zf2-tutorial, so it finds composer.json