chore(travis): only test against node 6
chore(travis): only test against node 6

Signed-off-by: Will Soto <e88b744add47cc3da30186f5601865928acd804d@gmail.com>

paradox41/angular-chartist.js,paradox41/angular-chartist.js,willsoto/angular-chartist.js,willsoto/angular-chartist.js