Remove non necessary files from bundled Ruby Gem

[rzhade3]

The current bundled Rubygem contains all the files in this directory, bloating the size of the gem and causing unnecessary changes when the only update to this repo was a Actions configuration update.

We should instead update the .gemspec to only bundle necessary files into the gem. This should be done by updating this line:

secure_headers/secure_headers.gemspec

Line 18 in b134eef

gem.files = `git ls-files`.split($INPUT_RECORD_SEPARATOR)