Debian build system is not a git repo

--- a/spamcheck.gemspec
+++ b/spamcheck.gemspec
@@ -14,7 +14,7 @@
   spec.homepage      = "https://gitlab.com/gitlab-org/gl-security/engineering-and-research/automation-team/spam/spamcheck"
   spec.license       = "MIT"
 
-  spec.files         = `git ls-files -z #{prefix}`.split("\x0").reject { |f| f.match(%r{^#{prefix}/(test|spec|features)/}) }
+  spec.files         = Dir['ruby/**/*']
   spec.require_paths = [prefix]
 
   spec.add_dependency "grpc", "~> 1.0"