Fix a warning about deprecated fixnum

[alexwlchan]

I get the following warning when I run make html with greengloves/alexwlchan.net:30:

/usr/local/bundle/gems/bit-struct-0.15.0/lib/bit-struct.rb:3: warning: constant ::Fixnum is deprecated

The ico gem hasn't been updated in over five years, so it's probably abandoned. I use it in one place – to create favicons in the .ico format – and since I've added ImageMagick since I wrote that plugin, I should probably just remove the gem.