Fix backwards compatibility issue.

Environment could be a String or Proc

lib/puma/configuration.rb

@@ -200,7 +200,7 @@ def config_files
       return [] if files == ['-']
       return files if files.any?
 
-      first_default_file = %W(config/puma/#{@options[:environment].call}.rb config/puma.rb).find do |f|
+      first_default_file = %W(config/puma/#{environment_str}.rb config/puma.rb).find do |f|
         File.exist?(f)
       end
 
@@ -264,6 +264,10 @@ def environment
       @options[:environment]
     end
 
+    def environment_str
+      environment.respond_to?(:call) ? environment.call : environment
+    end
+
     def load_plugin(name)
       @plugins.create name
     end

test/test_config.rb

@@ -126,6 +126,15 @@ def test_config_files_with_rack_env
     end
   end
 
+  def test_config_files_with_specified_environment
+    conf = Puma::Configuration.new do
+    end
+
+    conf.options[:environment] = 'fake-env'
+
+    assert_equal ['config/puma/fake-env.rb'], conf.config_files
+  end
+
   def teardown
     FileUtils.rm_r("config/puma")
   end