ci: disable caching

[stephenplusplus]

We're getting some errors related to caching from Travis:

store build cache
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

https://travis-ci.org/GoogleCloudPlatform/google-cloud-node/jobs/163750414

store build cache
running `casher push` took longer than 180 seconds and has been aborted

https://travis-ci.org/GoogleCloudPlatform/google-cloud-node/jobs/163750411

I think with our recent improvements on unifying dependencies, we should be able to have decent per-build times. If not, we can always revert.