Disable Travis caching
Disable Travis caching

We are seeing lots of timeouts when trying to restore dependencies
from the cache. For these larger files the cache probably doesn't
really buy us much so disabling cache for now. The dependencies will
be downloaded directly from data.kitware.com.

OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz