Add require stringio

[bongole]

Fix StringIO Exception when using serverless offline with simple rack application.

Steps to reproduce

• Gemfile

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '~> 2.7'

gem 'rack'

• config.ru

class TestApp
  def call(env)
    status  = 200
    headers = { "Content-Type" => "text/html" }
    body    = ["Hello World!"]

    [status, headers, body]
  end
end

run TestApp.new

• serverless.yml

service: sls-test
frameworkVersion: '3'

provider:
  name: aws
  runtime: ruby2.7

functions:
  api:
    handler: rack_adapter.handler
    events:
      - http: GET /

plugins:
  - serverless-rack
  - serverless-offline

$ sls rack install
$ sls offline
$ curl http://localhost:3000/dev
...
GET /dev (λ: api)
✖ Command failed with exit code 1: ruby /home/bongole/dev/sls-test/node_modules/serverless-offline/dist/lambda/handler-runner/ruby-runner/invoke.rb rack_adapter handler
  /home/bongole/dev/sls-test/serverless_rack.rb:100:in `build_environ': uninitialized constant StringIO (NameError)
        from /home/bongole/dev/sls-test/serverless_rack.rb:227:in `handle_request'
        from /home/bongole/dev/sls-test/rack_adapter.rb:63:in `handler'
        from /home/bongole/dev/sls-test/node_modules/serverless-offline/dist/lambda/handler-runner/ruby-runner/invoke.rb:83:in `<main>'

[logandk]

Thanks, @bongole!