[BlocksKit] Only require MessageUI on iOS.

BlocksKit/1.0.0/BlocksKit.podspec

@@ -9,8 +9,10 @@ Pod::Spec.new do |s|
   s.source   = { :git => 'https://github.com/zwaldowski/BlocksKit.git', :tag => 'v1.0.0' }
   s.source_files = 'BlocksKit'
   s.dependency = 'A2DynamicDelegate'
-  s.frameworks = 'MessageUI'
   s.clean_paths = 'GHUnitIOS.framework/', 'Tests/', 'BlocksKit.xcodeproj/', '.gitignore'
+  if config.ios?
+    s.frameworks = 'MessageUI'
+  end
   def s.post_install(target)
     prefix_header = config.project_pods_root + target.prefix_header_filename
     prefix_header.open('a') do |file|