[CircleCi] add more soft fallbacks

[wilzbach]

this should set master as default branch in case we run into issues with the Github API rate limit.

@MartinNowak do you have better ideas? can we maybe expose a oauth token of a dummy account, s.t. we have a higher rate limit?

e.g. setting https://circleci.com/gh/dlang/dmd/edit#env-vars

and then using one of the variants here

curl -u "$OAUTH-TOKEN:x-oauth-basic" https://api.github.com/user
curl -H "Authorization: token $OAUTH-TOKEN" https://api.github.com
curl https://api.github.com/?access_token=$OAUTH-TOKEN

[dlang-bot]

@wilzbach, thanks for your PR! By analyzing the annotation information on this pull request, we identified @MartinNowak to be a potential reviewer

[WalterBright]

I'd pull this, but I have no experience with this sort of thing.