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.

Makefile

@@ -32,8 +32,8 @@ BTCD_COMMIT := $(shell cat go.mod | \
 
 GOACC_COMMIT := ddc355013f90fea78d83d3a6c71f1d37ac07ecd5
 
-GOBUILD := GO111MODULE=on go build -v
-GOINSTALL := GO111MODULE=on go install -v
+GOBUILD := GO111MODULE=on go build -v -trimpath
+GOINSTALL := GO111MODULE=on go install -v -trimpath
 GOTEST := GO111MODULE=on go test -v
 
 GOFILES_NOVENDOR = $(shell find . -type f -name '*.go' -not -path "./vendor/*")