git is not present in debian build environment

--- a/chronic.gemspec
+++ b/chronic.gemspec
@@ -13,8 +13,8 @@
   s.license = 'MIT'
   s.rdoc_options = ['--charset=UTF-8']
   s.extra_rdoc_files = %w[README.md HISTORY.md LICENSE]
-  s.files = `git ls-files`.split($/)
-  s.test_files = `git ls-files -- test`.split($/)
+  s.files = Dir['lib/**/*.rb']
+  s.test_files = Dir['test/**/*.rb']
 
   s.add_runtime_dependency 'numerizer', '~> 0.2'