Update dateformatter.js

Corrected date calculation for day of week.

it looks like getDay() returns 0-6 but that 0 is sunday. So no modifier is necessary for the other weekdays - we just need sunday to become 7.

Both of these print 7 right now
```
{{ '28 Dec 2013'|date('N') }}
{{ '29 Dec 2013'|date('N') }}
```