added simplecov for test coverge report

Guardfile

@@ -15,7 +15,7 @@
 #
 # and, you'll have to watch "config/Guardfile" instead of "Guardfile"
 
-guard :minitest, spring: "bin/rails test", all_after_pass: true, all_on_start: false do
+guard :minitest, spring: true, all_after_pass: true, all_on_start: false do
   watch(%r{^test/test_helper\.rb$}) { 'test' }
   watch(%r{^test/(.*)\/?(.*)_test\.rb$})