Fix podspec to remove duplicate symbol link errors

- Changed the podspec to eliminate Xcode 5 linker errors due to duplication
  symbols.

NJKWebViewProgress.podspec

@@ -7,12 +7,11 @@ Pod::Spec.new do |s|
   s.authors      = { "ninjinkun" => "ninjin@mac.com" }
   s.source       = { :git => "https://github.com/ninjinkun/NJKWebViewProgress.git", :tag => "v#{s.version}" }
   s.platform     = :ios, '4.3'
-  s.source_files = 'NJKWebViewProgress'
   s.requires_arc = true
   s.subspec 'Core' do |ss|
     ss.source_files = 'NJKWebViewProgress/NJKWebViewProgress.{h,m}'
   end
   s.subspec 'ProgressView' do |ss|
     ss.source_files = 'NJKWebViewProgress/NJKWebViewProgressView.{h,m}'
   end
-end
+end