Issue #872: add json/allocator.h in the amalgamated header.

I don't know why we didn't include this before.
It seems to work fine.

Author: BillyDonahue

Diff for: amalgamate.py

@@ -67,7 +67,7 @@ def amalgamate_source(source_top_dir=None,
     header.add_text("/// to prevent private header inclusion.")
     header.add_text("#define JSON_IS_AMALGAMATION")
     header.add_file("include/json/version.h")
-    #header.add_file("include/json/allocator.h") # Not available here.
+    header.add_file("include/json/allocator.h")
     header.add_file("include/json/config.h")
     header.add_file("include/json/forwards.h")
     header.add_file("include/json/features.h")