Using `go get` instead of `go get -d` + `go build`

as the latter [discards artifacts when multiple packages are specified](http://golang.org/cmd/go/#hdr-Compile_packages_and_dependencies) without an `-o` flag, which is the case when passing `./...`.

I also snuck in a `gobuild_flags` config var as I'd very much like to use the non-Makefile Travis build when possible, but would like to see the build output via the `-x` flag.  I'd also love to extend such a config var so that build matrix-like behavior is possible given the `go build` tool's support for build tags.  If such a config flag is acceptable, I'll gladly document it wherever appropriate.