Only check if the source of a pod is ‘local’ if there is a source.

Which might not be the case if a pod is part of another pod.

Fixes #130.

bin/pod

 #!/usr/bin/env ruby
 
 if $0 == __FILE__
+  $:.unshift File.expand_path('../../external/xcodeproj/ext', __FILE__)
   $:.unshift File.expand_path('../../external/xcodeproj/lib', __FILE__)
   $:.unshift File.expand_path('../../lib', __FILE__)
 end

lib/cocoapods/specification.rb

@@ -154,7 +154,7 @@ module Pod
     include Config::Mixin
     
     def local?
-      !source[:local].nil?
+      !source.nil? && !source[:local].nil?
     end
     
     def local_path