[Source Aggregate] Use select not keep_if

select returns a new array and keep_if isn't available in old ruby

lib/cocoapods-core/source/aggregate.rb

@@ -109,7 +109,7 @@ def search_by_name(query, full_text_search = false)
 
         root_spec_names = pods_by_source.values.flatten.uniq
         root_spec_names.each do |pod|
-          result_sources = sources.keep_if do |source|
+          result_sources = sources.select do |source|
             pods_by_source[source].include?(pod)
           end