README: point travis badges to fdroidtravis until f-droid mirror returns

@patcon setup https://github.com/f-droid to mirror the official gitlab
repos.  For some unknown reason, Travis stopped building new commits coming
into that repo about 3 months ago.  This switches to a github.com that I
manually push to, and Travis still builds.

Please switch back to the automatic mirror once it works again.
parent 78cd2b84
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