Ignore .pem files

Ignore certificate files created by running stack.sh.

Change-Id: Ib11e87261dad5985e7852c339243035d715924ed
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
diff --git a/.gitignore b/.gitignore
index c5744b3..17cb38c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,3 +9,4 @@
 files/*.gz
 files/images
 stack-screenrc
+*.pem