build: bump travis build go version to 1.13
build: bump travis build go version to 1.13

Change-Id: Ia666b638738b7a70519a58df16b1fe6dda101731
Reviewed-on: https://upspin-review.googlesource.com/c/augie/+/19388
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/augie,upspin/augie,upspin/augie