requirements.txt: Use coala IGitt remote as dependency

The gitmate IGitt repository was bit outdated, and was
using older version of GitHub api requests management.
Since, GitHub updated there api endpoints by depreciating
access token in query params, and recommended to pass
query params in request headers. So, to avoid any future
breaks related to GitHub api, updated coala Gitmate IGitt
repo with updated github token api headers.

Author: KVGarg

requirements.txt

@@ -4,7 +4,7 @@ git-url-parse
 django>2.1,<2.2
 django-distill
 django-eventtools
-git+https://gitlab.com/gitmate/open-source/IGitt.git@1fa5a0a21ea4fb8739d467c06972f748717adbdc
+git+https://gitlab.com/coala/gitmate/IGitt.git
 requests
 git+https://github.com/andrewda/trav.git@ce805d12d3d1db0a51b1aa26bba9cd9ecc0d96b8
 python-dateutil