Allow usage of simplecov with START_SIMPLECOV=1

Author: flori

Gemfile

@@ -5,3 +5,5 @@ source 'https://rubygems.org'
 gemspec :name => 'json'
 gemspec :name => 'json_pure'
 gemspec :name => 'json-java'
+
+gem 'simplecov'

tests/test_helper.rb

@@ -17,3 +17,7 @@
   require 'byebug'
 rescue LoadError
 end
+if ENV['START_SIMPLECOV'].to_i == 1
+  require 'simplecov'
+  SimpleCov.start
+end