Updated travis configuration to use spark-0.8.1
Updated travis configuration to use spark-0.8.1

poolio/thunder,kcompher/thunder,zhwa/thunder,poolio/thunder,pearsonlab/thunder,jwittenbach/thunder,oliverhuangchao/thunder,broxtronix/thunder,thunder-project/thunder,mikarubi/thunder,mikarubi/thunder,kunallillaney/thunder,j-friedrich/thunder,zhwa/thunder,kunallillaney/thunder,oliverhuangchao/thunder,kcompher/thunder,pearsonlab/thunder,j-friedrich/thunder,broxtronix/thunder