Gemfile: use HTTPS for the RubyGems source (#180)

The gem source was declared as `http://rubygems.org`, so `bundle
install` fetched dependency metadata and gem archives over plaintext
HTTP. That connection is neither encrypted nor authenticated, letting an
on-path attacker substitute the index response and serve arbitrary gem
code, which Bundler unpacks and loads on developer machines and in CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Watson1978

Gemfile

@@ -1,2 +1,2 @@
-source "http://rubygems.org"
+source "https://rubygems.org"
 gemspec

Gemfile.v0.12

@@ -1,4 +1,4 @@
-source "http://rubygems.org"
+source "https://rubygems.org"
 
 gem 'json', '= 1.8.6'
 gem 'fluentd', '~> 0.12.0'