Add support for Ruby 4.0

Ruby 4.0 removed some gems from its standard library (e.g. ostruct),
used by Rack.  This is fixed in newer versions of Rack, but this require
a newer version of Sinatra, which is currently restricted in the
gemspec.

Bump the minimum version of Sinatra so that we have a version of Rack
that includ the fix.  Provide appropriate fix to the code, and also
include rackup and a webserver that used to be bundled with rack but is
not anymore in the Gemfile (so that other components can be used by end
users to serve riemann-dash).

While here, prevent usage of minitest 6.0 which has some breaking
changes we do not cope with (yet).

Author: smortex

.github/workflows/ci.yml

@@ -20,6 +20,7 @@ jobs:
           - "3.1"
           - "3.2"
           - "3.3"
+          - "4.0"
       fail-fast: false
     steps:
       - uses: actions/checkout@v2

Gemfile

@@ -4,3 +4,5 @@ source 'https://rubygems.org'
 
 gemspec
 
+gem 'puma'
+gem 'rackup'

lib/riemann/dash/rack/static.rb

@@ -2,7 +2,7 @@ class Riemann::Dash::Static
   def initialize(app, options = {})
     @app = app
     @root = options[:root] or raise ArgumentError, "no root"
-    @file_server = ::Rack::File.new(@root)
+    @file_server = ::Rack::Files.new(@root)
   end
 
   def call(env)

riemann-dash.gemspec

@@ -14,11 +14,11 @@ Gem::Specification.new do |gem|
   gem.platform      = Gem::Platform::RUBY
 
   gem.add_runtime_dependency 'erubi', '~> 1.9.0'
-  gem.add_runtime_dependency 'sinatra', '>= 1.4.5', '< 2.3.0'
+  gem.add_runtime_dependency 'sinatra', '>= 4.0.0'
   gem.add_runtime_dependency 'sass', '>= 3.1.14'
   gem.add_runtime_dependency 'webrick'
   gem.add_runtime_dependency 'multi_json', '1.3.6'
-  gem.add_development_dependency 'minitest'
+  gem.add_development_dependency 'minitest', '< 6.0'
   gem.add_development_dependency 'github_changelog_generator'
   gem.add_development_dependency 'rake'
   gem.files         = `git ls-files`.split($/)