Scrub redis argument to prevent encoding issues

Recently I noticed this in the slow log:

```
2014-11-18_18:00:54.51307 event="exception" class="ArgumentError" message="invalid byte sequence in US-ASCII" backtrace="[\"/usr/local/bin/redis-slowlog:18:in `initialize'\" ...
```

Since we only care to match against pure ASCII characters `scrub` is good enough. But it mean a Ruby 2.1 dependency though.