Installing PHARs fails when phar.readonly is set in php.ini

[sebastianheuer]

Installation of PHARs fails if the setting phar.readonly is set to On in php.ini (which is the default) and PHIVE has been installed as a PHAR.

$ phive install phpunit                                                                                               [9:20:56]
Downloading https://phar.phpunit.de/phive.xml
Downloading https://phar.phpunit.de/phpunit-5.3.1.phar
Downloading https://phar.phpunit.de/phpunit-5.3.1.phar.asc
[ERROR]    Undefined offset: 2
[ERROR]   An error occured while processing your request:

          DOMDocument::save(phar:///usr/local/bin/phive/src/../phive.xml): failed to open stream: phar error: write operations disabled by the php.ini setting phar.readonly

          #0 har:///usr/local/bin/phive/src/shared/repository/WritableXmlRepository.php(10)
          #1 unknown file(0): PharIo\Phive\Cli\Runner->errorHandler()
          #2 har:///usr/local/bin/phive/src/shared/repository/WritableXmlRepository.php(10): DOMDocument->save()
          #3 har:///usr/local/bin/phive/src/shared/config/PhiveXmlConfig.php(19): PharIo\Phive\WritableXmlRepository->save()
          #4 har:///usr/local/bin/phive/src/commands/install/InstallCommand.php(61): PharIo\Phive\PhiveXmlConfig->addPhar()
          #5 har:///usr/local/bin/phive/src/shared/cli/Runner.php(51): PharIo\Phive\InstallCommand->execute()
          #6 usr/local/bin/phive(141): PharIo\Phive\Cli\Runner->run()
          #7 {main}

          Environment: PHP 5.6.19
          Phive Version: 0.2.0

          This should not have happend and is most likely a bug.
          Please report it at https://github.com/phar-io/phive/issues, make sure you include
          the full output of this error message. Thank you!