Disable npm cache on Travis. (#4468)
Disable npm cache on Travis. (#4468)

Add TODO noting to clean up disk-cache separately.

Cloned from #4448 to land while OP is away.
PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs