Add an ignore rules for Redis

Ignore all redis binary dump files (default: dump.rdb).

These backup/crash generated files contain a binary representation of
the in-memory store redis store and should not be tracked in version
control.
Merge branch 'feature/redis' of github.com:adamrights/gitignore into feature/redis

* 'feature/redis' of github.com:adamrights/gitignore:
  Add a ignore rules for Redis