Fixed require issue.

Running:
```
bin/redis_analytics_dashboard --redis-host 127.0.0.1 --redis-port 6379 --redis-db 0 --redis-namespace rs  
```
Gave me this error:
```
redis_analytics/lib/redis_analytics/dashboard.rb:18:in `<class:Dashboard>': uninitialized constant Rack::RedisAnalytics::Helpers (NameError)
redis_analytics/lib/redis_analytics/dashboard.rb:11:in `<module:RedisAnalytics>'
redis_analytics/lib/redis_analytics/dashboard.rb:9:in `<module:Rack>'
redis_analytics/lib/redis_analytics/dashboard.rb:8:in `<top (required)>'