add dependency to base64 as removed from ruby 3.4 stdlib

[mathbruyen]

safe_yaml fails upon being required in Ruby 3.4 because base64 has been removed from stdlib.

0.810 /usr/local/bundle/gems/safe_yaml-1.0.5/lib/safe_yaml/transform.rb:1:in 'Kernel#require': cannot load such file -- base64 (LoadError)                                                                         
0.810   from /usr/local/bundle/gems/safe_yaml-1.0.5/lib/safe_yaml/transform.rb:1:in '<top (required)>'
0.810   from /usr/local/bundle/gems/safe_yaml-1.0.5/lib/safe_yaml/load.rb:22:in 'Kernel#require'
0.810   from /usr/local/bundle/gems/safe_yaml-1.0.5/lib/safe_yaml/load.rb:22:in '<top (required)>'