Fix python 3.6+ compatibility when iterating over babel dicts

[amatissart]

Hi! On python 3.6+, day names are offset by one.
Latest versions of python preserve insertion order in dicts, and Sunday comes first in Babel data for some reason. So it is necessary to sort them by key explicitly.

Why not setting up Travis and run the tests on multiple versions of Python automatically ?

[amatissart]

And that should fix #20 :)

[rezemika]

Thank you! I just rebased and merged it by hand. :)

[amatissart]

Thanks!