R1:69562319bec6

add Haskell extensions to ##differential.whitespace-matters## in default.conf.py

Summary: ##/\.l?hs$/## files are sensitive to changes in whitespace

Test Plan: what, me worry?

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1482