Update dashboard.js

Tinkering with the input box I set refresh interval to 0 and various negative numbers.  This would cause cpu load to spike, and the graphs would never load.  The time in the upper right would tick off every second, though I suspect it was ticking much faster but it only has 1-second resolution.  The first time I did this Firefox stopped behaving for ~2-3 minutes but eventually let me change it. I repeated this test using firebug and confirmed it was essentially refreshing the UI as fast as it could spawn new requests.  This diff ensures the refresh interval is a positive number greater than or equal to '1'.  I originally set it '>0' but the decided to see what would happen if I fed '0.1' via the UI and the bug came back.  This diff appears to work for me.