add_text: 'page' is now a mandatory argument

to fix SyntaxError: non-default argument follows default argument
https://travis-ci.org/wikimedia/pywikibot-core/jobs/31764485#L871

actually, there was no reason for making 'page' optional

Change-Id: I3de824dfd61276a3943b543db5d86f3ba3ce6b28
Merge "add_text: 'page' is now a mandatory argument"