Update .travis.yml to use xvfb
Update .travis.yml to use xvfb

Tails/vexflow,Huji/vexflow,aaronmars/vexflow,aaronmars/vexflow,aaronmars/vexflow,Tails/vexflow,Huji/vexflow,Silverwolf90/vexflow,Silverwolf90/vexflow,Tails/vexflow,Huji/vexflow,Silverwolf90/vexflow