build: build with Go 1.13 in Travis
build: build with Go 1.13 in Travis

Change-Id: Ic11b5e15ccfb47bcf1688b3a8ce79d0384897c75
Reviewed-on: https://upspin-review.googlesource.com/c/exp/+/19340
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/exp,upspin/exp