build: don't fetch dependencies in TravisCI; they should be vendored

We don't actually want to do a 'go get' for the upspin repo because all
its dependencies should be vendored. By disabling the TravisCI install
step we make sure TravisCI will fail if we introduce a dependency by
mistake.

Change-Id: I968537f36440050c0e61fa75ef2ee74febc99b57
Reviewed-on: https://upspin-review.googlesource.com/14742
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/.travis.yml b/.travis.yml
index 2755f7d..6fde049 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,7 @@
 go:
 - 1.9.x
 go_import_path: augie.upspin.io
+install: true
 script: go test -short -race ./...
 notifications:
   email: