.pullapprove.yml: Count rejections against

Distinguish between "I haven't looked at this" (no need to say
anything) and "I have looked at this and don't like it" (say something
matching the reject_regex).

Docs in http://docs.pullapprove.com/groups/reject_value/

Signed-off-by: W. Trevor King <wking@tremily.us>

.pullapprove.yml

@@ -6,6 +6,7 @@ requirements:
 
 group_defaults:
   required: 2
+  reject_value: -1
   approve_by_comment:
     enabled: true
     approve_regex: ^LGTM