GelfHandlerTest fails with an error when mlehner/gelf-php is not installed

[msabramo]

It looks like GelfHandlerTest has the same issue as pointed out by @stof in a code review for RavenHandler:

~/Sites/sentry-test/vendor/monolog/monolog$ mv vendor/mlehner vendor/_mlehner 
~/Sites/sentry-test/vendor/monolog/monolog$ ls -ld vendor/mlehner
ls: cannot access vendor/mlehner: No such file or directory
~/Sites/sentry-test/vendor/monolog/monolog$ phpunit
PHP Fatal error:  Class 'Gelf\MessagePublisher' not found in /Users/marca/Sites/sentry-test/vendor/monolog/monolog/tests/Monolog/Handler/GelfHandlerTest.php on line 21
PHP Stack trace:
PHP   1. {main}() /Users/marca/pear/bin/phpunit:0
PHP   2. PHPUnit_TextUI_Command::main() /Users/marca/pear/bin/phpunit:46
PHP   3. PHPUnit_TextUI_Command->run() /Users/marca/pear/share/pear/PHPUnit/TextUI/Command.php:130
PHP   4. PHPUnit_TextUI_Command->handleArguments() /Users/marca/pear/share/pear/PHPUnit/TextUI/Command.php:139
PHP   5. PHPUnit_Util_Configuration->getTestSuiteConfiguration() /Users/marca/pear/share/pear/PHPUnit/TextUI/Command.php:671
PHP   6. PHPUnit_Util_Configuration->getTestSuite() /Users/marca/pear/share/pear/PHPUnit/Util/Configuration.php:768
PHP   7. PHPUnit_Framework_TestSuite->addTestFiles() /Users/marca/pear/share/pear/PHPUnit/Util/Configuration.php:848
PHP   8. PHPUnit_Framework_TestSuite->addTestFile() /Users/marca/pear/share/pear/PHPUnit/Framework/TestSuite.php:419
PHP   9. PHPUnit_Util_Fileloader::checkAndLoad() /Users/marca/pear/share/pear/PHPUnit/Framework/TestSuite.php:358
PHP  10. PHPUnit_Util_Fileloader::load() /Users/marca/pear/share/pear/PHPUnit/Util/Fileloader.php:79
PHP  11. include_once() /Users/marca/pear/share/pear/PHPUnit/Util/Fileloader.php:95

Fatal error: Class 'Gelf\MessagePublisher' not found in /Users/marca/Sites/sentry-test/vendor/monolog/monolog/tests/Monolog/Handler/GelfHandlerTest.php on line 21

Call Stack:
    0.0003     633432   1. {main}() /Users/marca/pear/bin/phpunit:0
    0.0041    1150600   2. PHPUnit_TextUI_Command::main() /Users/marca/pear/bin/phpunit:46
    0.0042    1151328   3. PHPUnit_TextUI_Command->run() /Users/marca/pear/share/pear/PHPUnit/TextUI/Command.php:130
    0.0042    1151328   4. PHPUnit_TextUI_Command->handleArguments() /Users/marca/pear/share/pear/PHPUnit/TextUI/Command.php:139
    0.0164    3191376   5. PHPUnit_Util_Configuration->getTestSuiteConfiguration() /Users/marca/pear/share/pear/PHPUnit/TextUI/Command.php:671
    0.0164    3192408   6. PHPUnit_Util_Configuration->getTestSuite() /Users/marca/pear/share/pear/PHPUnit/Util/Configuration.php:768
    0.0232    3560352   7. PHPUnit_Framework_TestSuite->addTestFiles() /Users/marca/pear/share/pear/PHPUnit/Util/Configuration.php:848
    0.0596    6247784   8. PHPUnit_Framework_TestSuite->addTestFile() /Users/marca/pear/share/pear/PHPUnit/Framework/TestSuite.php:419
    0.0597    6248384   9. PHPUnit_Util_Fileloader::checkAndLoad() /Users/marca/pear/share/pear/PHPUnit/Framework/TestSuite.php:358
    0.0597    6248544  10. PHPUnit_Util_Fileloader::load() /Users/marca/pear/share/pear/PHPUnit/Util/Fileloader.php:79
    0.0600    6392056  11. include_once('/Users/marca/Sites/sentry-test/vendor/monolog/monolog/tests/Monolog/Handler/GelfHandlerTest.php') /Users/marca/pear/share/pear/PHPUnit/Util/Fileloader.php:95