Update Serbian timestamp wordings

[jimjohnsonrollings]

Is your feature request related to a problem? Please describe.
Some of the wording around timestamps is grammatically off in Serbian.

Describe the solution you'd like
Update the following in the psammead-locales package:

xMinutesAgo:
which currently is:
lat: Pre minuta: {x}
cyr: Пре минута: {x}
but it would be grammatically better if it were:
lat: Pre {x} minuta
cyr: Пре {x} минута

For the following however, it's more complex, as there is a need to use a different word for "Hours" when the number of hours is less than 5, versus the word used when the number of hours is 5 or more.

Investigate how this can be achieved with moment.js as we are using it currently - see https://stackoverflow.com/questions/38367038/format-relative-time-in-momentjs for an example.

to fix xHoursAgo which currently is:
lat: Pre sati: {x}
cyr: Пре сати: {x}
it would need to be:
for x < 5 :
lat: Pre {x} sata
cyr: Пре {x} сата
and for x >= 5 :
lat: Pre {x} sati
cyr: Пре {x} сати

Describe alternatives you've considered
N/A

Testing notes
Include this scenario in testing - published timestamp greater than and less than 5 hours.

Dev insight: Will there be any potential regression? etc

• This feature is expected to need manual testing.

Additional context
https://github.com/bbc/psammead/tree/latest/packages/utilities/psammead-locales/moment