Update transifex resource url

Transifex complained about the removal of the develop branch. If I'm not mistaken this is intentional? I updated the fetch url, now getting the strings to translate from https://raw.githubusercontent.com/leapcode/leap_web/master/lib/en_US.yml

If that's not ok, shout.

(from redmine: created on 2016-10-22, closed on 2016-10-22)