Debian build environment is not a git repo (we use tarballs)

--- a/googleauth.gemspec
+++ b/googleauth.gemspec
@@ -18,11 +18,9 @@
    https://developers.google.com/accounts/docs/application-default-credentials
   DESCRIPTION
 
-  gem.files         = `git ls-files`.split "\n"
-  gem.test_files    = `git ls-files -- spec/*`.split "\n"
-  gem.executables   = `git ls-files -- bin/*.rb`.split("\n").map do |f|
-    File.basename f
-  end
+  gem.files         = Dir.glob("**/*").select {|v| v !~ /^debian/}
+  gem.test_files    = gem.files.select {|v| v =~ /^(test|spec|features)/}
+  gem.executables   = gem.files.select {|v| v =~ /^bin\//}.map { |f| File.basename(f) }
   gem.require_paths = ["lib"]
   gem.platform      = Gem::Platform::RUBY
   gem.required_ruby_version = ">= 2.4.0"