travis: do not run tests in parallel
travis: do not run tests in parallel

Attempt to use less resources and avoid spurious failures.

mattnworb/helios,molindo/helios,skinzer/helios,DagensNyheter/helios,udomsak/helios,dflemstr/helios,kidaa/helios,gtonic/helios,DagensNyheter/helios,gtonic/helios,mbruggmann/helios,molindo/helios,mbruggmann/helios,spotify/helios,kidaa/helios,spotify/helios,gtonic/helios,mattnworb/helios,mavenraven/helios,udomsak/helios,skinzer/helios,stewnorriss/helios,skinzer/helios,mavenraven/helios,dflemstr/helios,mbruggmann/helios,mattnworb/helios,stewnorriss/helios,spotify/helios,mavenraven/helios,molindo/helios,udomsak/helios,kidaa/helios,dflemstr/helios,stewnorriss/helios,DagensNyheter/helios