[infra] update Travis cache with built webapp/ (part 3) (#4094)
[infra] update Travis cache with built webapp/ (part 3) (#4094)

Originally, all build targets were disabled [1] during CI in an
effort [2] to rebuild the cache.

This re-adds tensorboard/webapp/ to the list of targets to be
built by Travis CI.  Followups will add the remaining targets.

[1] https://github.com/tensorflow/tensorboard/pull/4092
[2] https://github.com/tensorflow/tensorboard/issues/4091
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard