Changed to prefer dist

[baileyparker]

Addresses Issue #19 involving --prefer-source over --prefer-dist.

[briankiewel]

Unfortunately I think it's necessary to use --prefer-source because you can occasionally run into rate limiting with Github's HTTP API since all Pagodabox deploys are originating from the same place and Github can't differentiate between sources so the hourly limit can get used up relatively quickly. I believe --prefer-source will use git commands to pull it thereby bypassing the HTTP API rate limits.

I had a few discussions with Pagodabox regarding this, but also here's a couple of related links.

http://help.pagodabox.com/customer/portal/questions/6193893-failed-to-download-doctrine-common-from-dist-could-not-authenticate-against-github-com

composer/composer#1861

[baileyparker]

That's very interesting. Thanks for sharing those. I was completely unaware that --prefer-dist was subject to API rate limits, although now that I think about it, it does make sense. Apologies for PR'ing without doing my research.