travis: upgrade Go 1.8 to 1.8.1
travis: upgrade Go 1.8 to 1.8.1

Go 1.8.1. has been released recently. Use it for the Go 1.8 branch.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs