[pod-list] avoid exception usage for control flow

lib/cocoapods/command/list.rb

@@ -38,11 +38,13 @@ module Pod
       end
 
       def last_check_time
+        if File.exists?(last_check_file)
           string = File.open(last_check_file, "rb").read
           Time.parse(string)
-      rescue
+        else
           Time.now - 60 * 60 * 24 * 15
         end
+      end
 
       def new_sets_since(time)
         all = Source.all_sets