.travis.yml: build with Go 1.9rc2 too.

.travis.yml

@@ -5,6 +5,7 @@ language: go
 
 go:
   - 1.8.3
+  - 1.9rc2
   - tip
 
 # Use the virtualized Trusty beta Travis is running in order to get support for
@@ -50,4 +51,7 @@ install: true
 script:
   - cd $GOPATH/src/github.com/
   - mv GoogleCloudPlatform googlecloudplatform
-  - go test -v github.com/googlecloudplatform/gcsfuse/...
+  # Set GOROOT to work around https://github.com/travis-ci/travis-ci/issues/8229.
+  # TODO(jacobsa): once that issue is fixed, remove the workaround and make
+  # sure the build still passes with Go 1.9.
+  - GOROOT=$(go env GOROOT) go test -v github.com/googlecloudplatform/gcsfuse/...