Fixed composer.json for correct autoload

The original composer.json generated the following error:
  [RuntimeException]                                                                                                                                                                          
  Could not scan for classes inside "/home/dromanin/NetBeansProjects/google-test/vendor/google/api-client/vendor/google-api-php-client/src/" which does not appear to be a file nor a folder  

The autoload classmap path should be relative to composer.json position, src/ is enough.

Regards,
Davide Romanini