Update Ruby MCP client example (#177)

Follow-up to #121.

This PR removes the `base64` dependency from the Gemfile, as it became a runtime dependency of
Claude SDK for Ruby v1.60.0+.
anthropics/anthropic-sdk-ruby#161 (comment)

Author: koic

mcp-client-ruby/Gemfile

@@ -2,7 +2,6 @@
 
 source "https://rubygems.org"
 
-gem "anthropic"
-gem "base64"
+gem "anthropic", '>= 1.60.0'
 gem "dotenv"
 gem "mcp", '>= 0.15.0'