Add SMTP configuration to example .env
Add nokogiri as a dependency

Premailer requires nokogiri or hpricot as a dependency. It includes
neither as it lets the developer to decide which they prefer so we must
include one, I choose nokogiri.
Don't require raven

We should only require raven if a sentry url is provided which we're
doing in config/raven.rb. This removes it from being required by
bundler.
Merge pull request #254 from opscode/fix_premailer

Fix premailer