Added the dump.rdb file to gitignore

Whenever i'm working on GitLab I have redis running, but redis is
generating a db dump file called dump.rdb, I dont think that should be
checked in, so it should be on the gitignore list
Merge pull request #6706 from jvanbaarsen/add-dump-rdb-to-ignore

Added the dump.rdb file to gitignore