remove Travis CI status badge from README

[pmeier]

Although we migrated all remaining workflows from Travis CI to CIrcleCi in #3137 the status badge in the README was not upgraded:

This PR replaces this with

This is the default badge style from CircleCI, but we could also go with

for consistency.

[pmeier]

After some more research, it seems that the CircleCI badge is not reliable. Without knowing the internals, I imagine it does something like a logical OR instead of AND with the individual workflows.

There seem to be a lot of discussion about this in their support forums. As of now I think we should not place this badge since it might be misleading. @fmassa?

[pmeier]

I've contacted the CircleCI support about this. The badge only shows the status of the last run workflow which is on the top of the list in their UI, i.e. nightly. There is now a feature request for what we need. Until then we have only two options if we want the status badge:

1. Determine the most important workflow and move it to run last.
2. Merge all workflows into one.

IMO both options are subpar. Thus, I suggest we remove the badge for now and only re-add it if the feature request went through.

[fmassa]

Thanks @pmeier!

[fmassa]

I agree with your current solution, we can add the CircleCI badge in the future