Change the value of source in Gemfile because deprecated

Tatsuya Sato · Tatsuya Sato · commit fe95779fac9e · 2013-04-12T20:54:07.000+09:00
diff --git a/Gemfile b/Gemfile
@@ -1,2 +1,2 @@
-source :rubygems
+source 'https://rubygems.org'
 gemspec
