fix cache path
fix cache path

the path used seems to be `/tmp/vagrant-cache`. at least in 0.7.2
BayanGroup/vagrant-cachier,bbatsche/vagrant-cachier,raghavendra-talur/vagrant-cachier,bbatsche/vagrant-cachier,patcon/vagrant-cachier,raghavendra-talur/vagrant-cachier,aaray/vagrant-cachier,patcon/vagrant-cachier,JulianVolodia/vagrant-cachier,BayanGroup/vagrant-cachier,fgrehm/vagrant-cachier,aaray/vagrant-cachier,JulianVolodia/vagrant-cachier,fgrehm/vagrant-cachier