Debian package source tree is not a git repo.

--- a/rspec-rails.gemspec
+++ b/rspec-rails.gemspec
@@ -21,7 +21,7 @@
     'source_code_uri'   => 'https://github.com/rspec/rspec-rails',
   }
 
-  s.files            = `git ls-files -- lib/*`.split("\n")
+  s.files            = Dir.glob("lib/**/*")
   s.files           += %w[README.md LICENSE.md Changelog.md Capybara.md .yardopts .document]
   s.test_files       = []
   s.rdoc_options     = ["--charset=UTF-8"]