This updates the Go versions using when running the TravisCI integration
tests to reflect the latest supported Go versions.
Also, the vet tool moved into the Go source tree as of Go 1.5. Its previous
location in the x/tools repo was deprecated at that time and has now
been removed.
Finally, old tool path is no longer needed, so it has been removed.
Also update to use the new container-based builds
For now, golint has been disabled since it fails due to wanting
HTTPPostMode. This change will be made as a part of the upgrade to use
btcjson2 since it involves an API change.
