A fix for Ukrainian (uk) language translation

Made amendments to Ukrainian translation to fix the issue with displaying calendar dates like this:
```javascript
moment().subtract('days', 3).calendar();    //"В минулу середа в 23:05" ("Last Wednesday at 11:05 PM") 
```  
In the example above 'середа' is in nominative instead of the needed accusative ('середу').

Fixed by changing the format of "Last %weekday%" expression in Ukrainian to the more grammatically accurate, which uses genitive case, like in the example below:
```javascript
moment().subtract('days', 3).calendar();    //"Минулої середи в 23:05"
```