[locale] (sw) Improve Translation (#5546)

[locale] (sw) Improve Translation (#5546)

* [locale] (sw) Improve Translation

It appears that "masiku" does not usually have a plural , going back to "siku" ,
Other propositions from https://github.com/moment/moment/pull/5544 sounds a bit nice and native,

 But, sadly we cannot have them since they are likely to change on future Relative Time , so it is better if they stay that way

* [locale] (sw) Improve Translation In Tests

This is so that the Travis CI builds https://github.com/moment/moment/pull/5546/commits