Make 0.4 to be default.

Author: AlexeyKuznetsov-DD

dd-trace-api/src/main/java/datadog/trace/api/ConfigDefaults.java

@@ -88,10 +88,7 @@ public final class ConfigDefaults {
       Arrays.asList("user.id", "session.id", "account.id");
   static final boolean DEFAULT_JMX_FETCH_ENABLED = true;
 
-  // TODO: forced to use 1.0 for testing only, rollback to "0.4" before merging to master;
-  static final String DEFAULT_TRACE_AGENT_PROTOCOL_VERSION = ProtocolVersion.V1_0.asConfigValue();
-  // TODO: static final String DEFAULT_TRACE_AGENT_PROTOCOL_VERSION =
-  // ProtocolVersion.V0_4.asConfigValue();
+  static final String DEFAULT_TRACE_AGENT_PROTOCOL_VERSION = ProtocolVersion.V0_4.asConfigValue();
 
   static final boolean DEFAULT_CLIENT_IP_ENABLED = false;