build: update travis to build against go 1.13

In this commit, we update Travis to start building against the newly
released go 1.13. Additionally, we'll now utilize the new `trimpath` to
the `go build` and `go install` commands. This new flag serves to remove
all file system paths from the compiled Go executable, which will make
our binaries more reproducible.

go.mod

@@ -50,3 +50,5 @@ replace github.com/lightningnetwork/lnd/ticker => ./ticker
 replace github.com/lightningnetwork/lnd/queue => ./queue
 
 replace git.schwanenlied.me/yawning/bsaes.git => github.com/Yawning/bsaes v0.0.0-20180720073208-c0276d75487e
+
+go 1.13