amalgamate: add version.h and allocator.h to the forwards header

Required to get JSONCPP_USING_SECURE_MEMORY and the SecureAllocator
available for the definition of Allocator.
diff --git a/amalgamate.py b/amalgamate.py
index fedee1e..4a328ab 100755
--- a/amalgamate.py
+++ b/amalgamate.py
@@ -99,6 +99,8 @@
     header.add_text("/// If defined, indicates that the source file is amalgamated")
     header.add_text("/// to prevent private header inclusion.")
     header.add_text("#define JSON_IS_AMALGAMATION")
+    header.add_file(os.path.join(INCLUDE_PATH, "version.h"))
+    header.add_file(os.path.join(INCLUDE_PATH, "allocator.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "config.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "forwards.h"))
     header.add_text("#endif //ifndef JSON_FORWARD_AMALGAMATED_H_INCLUDED")