Update TextSecure envelope protobuf.

Support envelope 'content' field.

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/entities/IncomingMessage.java

@@ -34,9 +34,11 @@ public class IncomingMessage {
   private int destinationRegistrationId;
 
   @JsonProperty
-  @NotEmpty
   private String body;
 
+  @JsonProperty
+  private String content;
+
   @JsonProperty
   private String relay;
 
@@ -67,4 +69,8 @@ public class IncomingMessage {
   public int getDestinationRegistrationId() {
     return destinationRegistrationId;
   }
+
+  public String getContent() {
+    return content;
+  }
 }