Fix RangeError when indents is negative

I was getting this error:
```
/usr/local/Cellar/node/0.8.12/lib/node_modules/mocha/lib/reporters/spec.js:33
    return Array(indents).join('  ')
           ^
RangeError: Invalid array length
    at indent (/usr/local/Cellar/node/0.8.12/lib/node_modules/mocha/lib/reporters/spec.js:33:12)
```

It turns out that the `indents` var was getting set to a negative number and causes this error. The fix was just to set it to 0 if it is below 0.