[Outdated] Fix crash that occurs every time pod outdated is run

Closes https://github.com/CocoaPods/CocoaPods/issues/2624.

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides/installing_cocoapods.html).
 
+## Master
+
+##### Bug Fixes
+
+* Fixes a crash when running `pod outdated`.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#2624](https://github.com/CocoaPods/CocoaPods/issues/2624)
+
+
 ## 0.34.2
 
 ##### Enhancements

lib/cocoapods/command/outdated.rb

@@ -98,7 +98,7 @@ module Pod
 
       def spec_sets
         @spec_sets ||= begin
-          aggregate = Source::Aggregate.new(analyzer.sources)
+          aggregate = Source::Aggregate.new(analyzer.sources.map(&:repo))
           installed_pods.map do |pod_name|
             aggregate.search(Dependency.new(pod_name))
           end.compact.uniq