Bump penlight from 1.7.0 to 1.31.1

[tkan145]

What

This commit also removes filesystem.lua as the issue has been fixed in penlight v1.8.0

https://github.com/lunarmodules/Penlight/pull/329

The dev image is already run with version 1.13.1 https://github.com/3scale/APIcast/blob/master/gateway/Roverfile.lock#L29

Verification Steps

• Build docker image from this git branch

  make runtime-image IMAGE_NAME=apicast-test

• HTTP dev environment setup

  cd dev-environments/plain-http-upstream
  make gateway IMAGE_NAME=apicast-test

  The gateway should be running without error

• The request should be accepted (200 OK)

  curl --resolve get.example.com:8080:127.0.0.1 -v "http://get.example.com:8080/?user_key=123"

[tkan145]

Added CHANGELOG entry. Because there is a new commit in PR, I need a new approval.