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README: Switch to CircleCI build status badge

We don't use TravisCI for regular builds anymore.
kidaa/helios,udomsak/helios,mattnworb/helios,skinzer/helios,DagensNyheter/helios,udomsak/helios,stewnorriss/helios,spotify/helios,gtonic/helios,mavenraven/helios,dflemstr/helios,gtonic/helios,DagensNyheter/helios,gtonic/helios,molindo/helios,dflemstr/helios,molindo/helios,dflemstr/helios,kidaa/helios,spotify/helios,mattnworb/helios,stewnorriss/helios,mavenraven/helios,mattnworb/helios,skinzer/helios,spotify/helios,kidaa/helios,skinzer/helios,udomsak/helios,molindo/helios,DagensNyheter/helios,stewnorriss/helios,mavenraven/helios