Provide faster compile time on travis
Provide faster compile time on travis

Apart of caching directories, we also use libsysconfcpus to compile it quicker. So elm-test runs on a warmed up cache.
ento/node-elm-test,rtfeldman/node-test-runner,rtfeldman/node-elm-test,ento/node-elm-test,rtfeldman/node-elm-test