Add go 1.12.x, remove go 1.8.x and 1.9.x
Add go 1.12.x, remove go 1.8.x and 1.9.x

[#163594716]

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci