fixing zone test for half hour offsets

This addresses problem 2 in #989, which is test failures that look like this:

```
>> zones - same / before / after
>> Message: two moments with different offsets should be the same hour
>> Error: two moments with different offsets should be the same hour
>> at Object.exports.zones.same / before / after (test/moment/zones.js:307:14)
>> at Object.exports.zones.setUp (test/moment/zones.js:6:9)

>> zones - same / before / after
>> Message: two moments with different offsets should be the same hour
>> Error: two moments with different offsets should be the same hour
>> at Object.exports.zones.same / before / after (test/moment/zones.js:308:14)
>> at Object.exports.zones.setUp (test/moment/zones.js:6:9)
```

The problem is that a lot of time zones are offset by a half-hour. A good list is here:

http://www.timeanddate.com/time/time-zones-interesting.html

The issue is what it means to say this:

```js
moment().zone(30).isSame(moment(), "hour");
```

You can see why it doesn't work with the current implementation, but after thinking about it for a while, I concluded that it's not really a meaningful question in the first place. It's just a question that only makes sense between zones with the same hour alignment. So I just changed the test.