travis: upgrade to go 1.12
travis: upgrade to go 1.12

The latest newt requires go 1.12+.

+go get mynewt.apache.org/newt/newt
../../../mynewt.apache.org/newt/newt/downloader/downloader.go:509:14: ee.ExitCode undefined (type *exec.ExitError has no field or method ExitCode)

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core