Update go to 1.11 in dockerfile
Update go to 1.11 in dockerfile

- Need go 1.11 to support go modules
- Unclear why we were using our own hosted version of go. Switching to
using the official source.

[#160699500]

Co-authored-by: An Yu <446e9838deaeb979640d56c46523b7c882d598cd@pivotal.io>

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