Fix CI with latest Rack

Older versions provided backwards compatibility, but with the new major
version of rack, this has gone away.

spec/riemann/tools/http_check_spec.rb

@@ -69,6 +69,14 @@ def protected!
 end
 
 RSpec.describe Riemann::Tools::HttpCheck, if: Gem::Version.new(RUBY_VERSION) >= Gem::Version.new(Riemann::Tools::HttpCheck::REQUIRED_RUBY_VERSION) do
+  # XXX: Needed for Ruby 2.6 compatibility
+  let(:rack_handler) do
+    if Object.const_defined?('Rackup::Handler::WEBrick')
+      Rackup::Handler::WEBrick
+    else
+      Rack::Handler::WEBrick
+    end
+  end
   describe '#endpoint_name' do
     subject { described_class.new.endpoint_name(address, port) }
 
@@ -108,7 +116,7 @@ def protected!
           Logger: WEBrick::Log.new(File.open(File::NULL, 'w')),
         }
         @server = WEBrick::HTTPServer.new(server_options)
-        @server.mount('/', Rack::Handler::WEBrick, TestWebserver)
+        @server.mount('/', rack_handler, TestWebserver)
         @started = false
         Thread.new { @server.start }
         Timeout.timeout(1) { sleep(0.1) until @started }
@@ -260,7 +268,7 @@ def protected!
           SSLCertName: '/CN=example.com',
         }
         @server = WEBrick::HTTPServer.new(server_options)
-        @server.mount('/', Rack::Handler::WEBrick, TestWebserver)
+        @server.mount('/', rack_handler, TestWebserver)
         @started = false
         Thread.new { @server.start }
         Timeout.timeout(1) { sleep(0.1) until @started }