Remove duplicated nodejs version in .travis.yml
Remove duplicated nodejs version in .travis.yml

lts/boron is v6

Yukaii/hackmd,PeterDaveHello/hackmd,Rwing/hackmd,Rwing/hackmd,hackmdio/hackmd,jackycute/HackMD,jackycute/HackMD,Yukaii/hackmd,PeterDaveHello/hackmd,Rwing/hackmd,hackmdio/hackmd,hackmdio/hackmd,PeterDaveHello/hackmd,Yukaii/hackmd,jackycute/HackMD