Change coverage output format to json

Author: Umekawa

spec/spec_helper.rb

@@ -1,7 +1,15 @@
 # frozen_string_literal: true
 
 require 'simplecov'
-SimpleCov.start
+require 'simplecov_json_formatter'
+
+SimpleCov.start do
+  formatter SimpleCov::Formatter::MultiFormatter.new([
+                                                       SimpleCov::Formatter::JSONFormatter,
+                                                       SimpleCov::Formatter::HTMLFormatter
+                                                     ])
+  add_filter '/spec/'
+end
 
 require 'ruboty/github'
 require 'webmock/rspec'