gems: drop foreman gem.

Foreman is unmaintained and locked to a really old version of thor,
which prevents upgrading to Rails 6.

The Procfile can still be used by manually installing Foreman with
`gem install foreman`. This is what Foreman's README recommends anyway.

bin/foreman

@@ -1,29 +0,0 @@
-#!/usr/bin/env ruby
-# frozen_string_literal: true
-
-#
-# This file was generated by Bundler.
-#
-# The application 'foreman' is installed as part of a gem, and
-# this file is here to facilitate running it.
-#
-
-require "pathname"
-ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
-
-bundle_binstub = File.expand_path("../bundle", __FILE__)
-
-if File.file?(bundle_binstub)
-  if File.read(bundle_binstub, 150) =~ /This file was generated by Bundler/
-    load(bundle_binstub)
-  else
-    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
-Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
-  end
-end
-
-require "rubygems"
-require "bundler/setup"
-
-load Gem.bin_path("foreman", "foreman")