Minor : add depency on 'thin' in gemspec

[mryan43]

rackup private_pub.ru -s thin -E production

doesnt work if the 'thin' gem ist not installed

i'm using gem 'rails', '3.2.9'

[nfm]

@m-ryan Faye can be used with a number of servers - thin, rainbows, and goliath. As such I think it would be better not to add a server dependency to the gemspec. I think it would be more appropriate to update the documentation.

[ryanb]

Good idea. Updated the readme to add "thin" to Gemfile.