Different way of excluding spec files

Ensures all test related files are not included.

httparty.gemspec

@@ -20,8 +20,11 @@ Gem::Specification.new do |s|
   # If this line is removed, all hard partying will cease.
   s.post_install_message = "When you HTTParty, you must party hard!"
 
-  s.files         = `git ls-files | grep -v ^spec`.split("\n")
-  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
+  all_files = `git ls-files`.split("\n")
+  test_files = `git ls-files -- {test,spec,features}/*`.split("\n")
+
+  s.files         = all_files - test_files
+  s.test_files    = test_files
   s.executables   = `git ls-files -- bin/*`.split("\n").map { |f| File.basename(f) }
   s.require_paths = ["lib"]
 end