git is not available in debian build environment

--- a/mini_histogram.gemspec
+++ b/mini_histogram.gemspec
@@ -18,9 +18,8 @@
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
-  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
-    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
-  end
+  all_files          = Dir.glob("**/*").select {|v| v !~ /^debian/}
+  spec.files         = all_files.select {|v| v =~ /^(test|spec|features)/}
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]