Remove `foreman` from Gemfile

https://github.com/ddollar/foreman/pull/437#issuecomment-41110407
Add unicorn
Setup `config/unicorn.rb`

Use `dotenv` gem to read `.env` file and update it when the server
starts or restarts. Configure number of unicorn workers via `ENV`
variable.
Add unicorn deploy tasks
Update .env.sample [ci skip]