[MRG] update instruction to install using ubuntu packages

Implements option (1) from #509 and address issue commented by https://github.com/scrapy/scrapy/issues/511#issuecomment-32687403

It also uses a new key for signing repository files:

    pub   2048R/627220E7 2014-01-18 [expires: 2024-01-16]
    uid                  Scrapy Team (APT Signing Key) <info@scrapy.org>

Previous repositories per codename still work, and they are kept uptodate and signed by old key:

    pub   2048R/19403F1D 2010-06-10
    uid                  Insophia (Scrapy Package Signing Key) <info@insophia.com>