Improve Travis caching (#2441)
Improve Travis caching (#2441)

* travis: cleanup compile-devtools work for faster build times.
* add gems to travis cache.

tkadlec/lighthouse,ev1stensberg/lighthouse,umaar/lighthouse,GoogleChrome/lighthouse,deepanjanroy/lighthouse,ev1stensberg/lighthouse,ev1stensberg/lighthouse,umaar/lighthouse,arturmiz/lighthouse,mixed/lighthouse,mixed/lighthouse,tkadlec/lighthouse,wardpeet/lighthouse,arturmiz/lighthouse,umaar/lighthouse,arturmiz/lighthouse,arturmiz/lighthouse,GoogleChrome/lighthouse,tkadlec/lighthouse,deepanjanroy/lighthouse,deepanjanroy/lighthouse,umaar/lighthouse,GoogleChrome/lighthouse,wardpeet/lighthouse,wardpeet/lighthouse,wardpeet/lighthouse,GoogleChrome/lighthouse,wardpeet/lighthouse,tkadlec/lighthouse,deepanjanroy/lighthouse,arturmiz/lighthouse,ev1stensberg/lighthouse,deepanjanroy/lighthouse,mixed/lighthouse,mixed/lighthouse,GoogleChrome/lighthouse