README: Update CI "passing" icon to use CircleCI
README: Update CI "passing" icon to use CircleCI

This has been showing a very outdated "passing" status from the last CI
run on Travis before we switched to CircleCI.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester