Clean up Travis config - ARM build only?

[IAlibay]

Follow up to #3036 and #3040

Once #3040 is merged (and follow ups; #3046, etc...) most of our CI builds will have been migrated away from Travis. The only thing that isn't possible at this moment are ARM builds.

Thanks to @tylerjereddy we have a few more units to use on Travis, if we use them conservatively for exotic platforms we can probably stretch it out until an alternative solution can be found.

I would suggest here possibly considering either a merge-only or a cron arm build so we can check that things run as expected.

Note: the credit spending on Travis has been turned off for now, once the Travis config has been cleaned up we can re-enable it.

[IAlibay]

Travis CI cron job from #3149 has now been set to a weekly run and works (https://travis-ci.com/github/MDAnalysis/mdanalysis/builds/220027210s). We can re-consider as merge only later (I suspect we might want to avoid this at the moment as we are likely going to get a large volume of merges during the GSoC period). -- closing