Doctrine Bundle 2.4 support

[vuryss]

Doctrine bundle 2.4 came with some breaking changes, one of which is the default configuration for DBAL connections.

In \Trikoder\Bundle\OAuth2Bundle\DependencyInjection\TrikoderOAuth2Extension::prepend
we have to remove the 'connections' => null, line - cause it's adding no value, but breaking the compatibility with the following error:

  [Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]                                                                                                                                    
  Unrecognized option "connections" under "doctrine.dbal.connections.default". Available options are "MultipleActiveResultSets", "application_name", "auto_commit", "charset", "connectstring", "dbname", "dbname  
  _suffix", "default_dbname", "default_table_options", "driver", "driver_class", "host", "instancename", "keep_replica", "keep_slave", "logging", "mapping_types", "memory", "options", "override_url", "password  
  ", "path", "persistent", "platform_service", "pooled", "port", "profiling", "profiling_collect_backtrace", "profiling_collect_schema_errors", "protocol", "replicas", "schema_filter", "server", "server_versio  
  n", "service", "servicename", "sessionMode", "shard_choser", "shard_choser_service", "shard_manager_class", "shards", "slaves", "sslcert", "sslcrl", "sslkey", "sslmode", "sslrootcert", "unix_socket", "url",   
  "use_savepoints", "user", "wrapper_class".

[vuryss]

Fixed with doctrine 2.4.2, which reverted their changes in 2.4.1