Remove the Bundler warning

With the last versions of bundler you get this warning:

> The source :rubygems is deprecated because HTTP requests are insecure.Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.

I did just that but as the versions of multiple gems are not locked the Gemfile.lock was updated with 

+    i18n (0.6.2)
+    sass (3.2.6)
+    serve (1.5.2)
+      activesupport (~> 3.2.12)
+      rack (~> 1.5.2)
+      rack-test (~> 0.6.2)
+      tilt (~> 1.3.3)

In fact, if the idea of the repo is to serve as bootstrapping base for a project and not as a ready-to-deploy-app, Gemfile.lock should not be in the repository.