Add ability to set rails_env.

Fall back on rails_env followed by "production" if capistrano_resque_rails_env is not set.

Inspiration came from a deploy.rb that sets its rails_env to an environment different than what is needed when running resque workers. I didn't like the following solution:

    after "deploy:restart", "restart_resque"

    desc "Restart Resque using capistrano-resque."
    task "restart_resque" do
        set :rails_env, "environment needed for resque"
        resque.restart
        set :rails_env, "previous environment"
    end

If an alternative, simpler solution exists please let me know and I'll close the pull request.

Solution adapted from https://github.com/javan/whenever/blob/master/lib/whenever/capistrano/recipes.rb