add known phpcs exclusions to stickler.yml

These two sniffs are consistently violated in most files, but
due to stickler only looking at files modified in the current
commit, it sometimes passes. To avoid those, disable those two
rules for now.

.stickler.yml

@@ -1,6 +1,7 @@
 linters:
   phpcs:
     standard: PSR2
+    exclude: 'Squiz.Classes.ValidClassName,PSR1.Classes.ClassDeclaration'
   csslint:
     ignore: 'ids,known-properties'
 files: