doc: update curl usage in COLLABORATOR_GUIDE

Github now redirects the patch urls hosted on github.com, making it
necessary to enable curl redirect handling to obtain patches in the
merge process.

Example (before and after):
https://github.com/iojs/io.js/pull/1382.patch
https://patch-diff.githubusercontent.com/raw/iojs/io.js/pull/1382.patch

PR-URL: https://github.com/iojs/io.js/pull/1382


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>