added python3 to travis
added python3 to travis

dannon/pysam,nlhepler/pysam,dannon/pysam,mattions/pysam,brendanofallon/pysam,mattions/pysam,kyleabeauchamp/pysam,nlhepler/pysam,daler/pysam,mattions/pysam,kyleabeauchamp/pysam,daler/pysam,bioinformed/pysam,nlhepler/pysam,carolinux/pysam,mattions/pysam,nlhepler/pysam,brendanofallon/pysam,daler/pysam,pysam-developers/pysam,pysam-developers/pysam,carolinux/pysam,carolinux/pysam,pysam-developers/pysam,TyberiusPrime/pysam,pysam-developers/pysam,daler/pysam,brendanofallon/pysam,TyberiusPrime/pysam,nlhepler/pysam,carolinux/pysam,carolinux/pysam,dannon/pysam,bioinformed/pysam,kyleabeauchamp/pysam,TyberiusPrime/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,bioinformed/pysam,TyberiusPrime/pysam,brendanofallon/pysam,mattions/pysam,daler/pysam,bioinformed/pysam,brendanofallon/pysam,TyberiusPrime/pysam,dannon/pysam,bioinformed/pysam,dannon/pysam