Fixing podspec for ARC.

Author: kreeger

SKInnerShadowLayer.podspec

@@ -21,4 +21,5 @@ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLI
   s.source    = { :git => 'https://github.com/khanlou/SKInnerShadowLayer.git', :tag => 'v1.1' }
   s.source_files  = 'SKInnerShadowLayer/SKInnerShadowLayer.{h,m}'
   s.frameworks    = 'QuartzCore'
+  s.requires_arc = true
 end