travis: Build for OpenJDK
travis: Build for OpenJDK

Should work identically for both Oracle JDK and OpenJDK, but it'll be
interesting to see if there are any differences.

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