Add rubocop to PR template checklist (#193)

Author: Maxscores

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,6 +7,7 @@
 - I have made corresponding changes to the documentation,
 - I have added tests that prove my fix is effective or that my feature works,
 - New and existing unit tests pass locally with my changes ("bundle exec rake"),
+- Ruby style passes ("bundle exec rubocop"),
 - Title include "WIP" if work is in progress.
 
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