Remove CONTRIBUTING.md from spec files (#51)

This was breaking the build. 🤦🏻

Author: jgarber623

link-header-parser.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
   spec.license       = "MIT"
 
   spec.files         = Dir["lib/**/*"].reject { |f| File.directory?(f) }
-  spec.files        += %w[LICENSE CHANGELOG.md CONTRIBUTING.md README.md]
+  spec.files        += %w[LICENSE CHANGELOG.md README.md]
   spec.files        += %w[link-header-parser.gemspec]
 
   spec.require_paths = ["lib"]