[Command] Use a case statement in Pod::Command#report_error

Addresses https://github.com/CocoaPods/CocoaPods/commit/0c80af37f1fec0009ad63277309afa47ad22595f#commitcomment-7718723.

lib/cocoapods/command.rb

@@ -48,11 +48,12 @@ module Pod
     end
 
     def self.report_error(exception)
-      if exception.is_a?(Interrupt)
+      case exception
+      when Interrupt
         puts '[!] Cancelled'.red
         Config.instance.verbose? ? raise : exit(1)
-      elsif exception.is_a?(SystemExit)
-        raise exception
+      when SystemExit
+        raise
       else
         if ENV['COCOA_PODS_ENV'] != 'development'
           puts UI::ErrorReport.report(exception)