Silence `:rubygems` Bundler deprecation notice.

Message from Bundler before this change:

  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.

1 files changed, 2 insertions, 1 deletions

diff --git a/Gemfile b/Gemfile
index c1b379a..86160d0 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,4 +1,5 @@
-source :rubygems
+source 'https://rubygems.org'
+
 gemspec :name => "mixlib-shellout"
 
 group(:test) do