Allows emacs version to parse json

Fixes issue: https://github.com/igrigorik/vimgolf/issues/132
That looked awkward to me on first glance. This is the error I get if I don't use the defvar:
```
error in process filter: goto-char: Symbol's function definition is void: url-http-end-of-headers
```
I found another way to bring this variable into scope:
```
(declare (special url-http-end-of-headers))
```
I'll end up using this instead, unless there's a better option.