Removed dependence on git in gemspec

dropbox-api.gemspec

@@ -27,8 +27,8 @@ Gem::Specification.new do |s|
     s.add_development_dependency 'ruby-debug19'
   end
 
-  s.files         = `git ls-files`.split("\n")
-  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
-  s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
+  s.files         = Dir['**/*'].keep_if { |file| File.file?(file) }
+  s.test_files    = Dir['spec/**/*']
+  s.executables   = Dir['bin/*'].map{ |f| File.basename(f) }
   s.require_paths = ["lib"]
 end