Merge branch '2.7' into 2.8

* 2.7:
  [travis] Test on PHP 7.1
  [FrameworkBundle] Convert null prefix to an empty string in translation:update command

Author: nicolas-grekas

Command/TranslationUpdateCommand.php

@@ -119,8 +119,9 @@ protected function execute(InputInterface $input, OutputInterface $output)
         // load any messages from templates
         $extractedCatalogue = new MessageCatalogue($input->getArgument('locale'));
         $io->comment('Parsing templates...');
+        $prefix = $input->getOption('prefix');
         $extractor = $this->getContainer()->get('translation.extractor');
-        $extractor->setPrefix($input->getOption('prefix'));
+        $extractor->setPrefix(null === $prefix ? '' : $prefix);
         foreach ($transPaths as $path) {
             $path .= 'views';
             if (is_dir($path)) {