ignore whitespace in pem files [skip ci]

Makefile

@@ -44,7 +44,7 @@ doc:
 	cd doc && PYTHONPATH=.. make html
 
 whitespace:
-	! find . -not -path "./.eggs/*" -not -path "./greentest/htmlcov/*" -not -path "./greentest/.coverage.*" -not -path "./.tox/*" -not -path "*/__pycache__/*" -not -path "*.so" -not -path "*.pyc" -not -path "./.git/*" -not -path "./build/*" -not -path "./libev/*" -not -path "./gevent/libev/*" -not -path "./gevent.egg-info/*" -not -path "./dist/*" -not -path "./.DS_Store" -not -path "./c-ares/*" -not -path "./gevent/gevent.*.[ch]" -not -path "./gevent/corecext.pyx" -not -path "./doc/_build/*" -not -path "./doc/mytheme/static/*" -type f | xargs egrep -l " $$"
+	! find . -not -path "*.pem" -not -path "./.eggs/*" -not -path "./greentest/htmlcov/*" -not -path "./greentest/.coverage.*" -not -path "./.tox/*" -not -path "*/__pycache__/*" -not -path "*.so" -not -path "*.pyc" -not -path "./.git/*" -not -path "./build/*" -not -path "./libev/*" -not -path "./gevent/libev/*" -not -path "./gevent.egg-info/*" -not -path "./dist/*" -not -path "./.DS_Store" -not -path "./c-ares/*" -not -path "./gevent/gevent.*.[ch]" -not -path "./gevent/corecext.pyx" -not -path "./doc/_build/*" -not -path "./doc/mytheme/static/*" -type f | xargs egrep -l " $$"
 
 pep8:
 	${PYTHON} `which pep8` .