Remove support for golang versions < 1.5
Remove support for golang versions < 1.5

[#132357135]

Signed-off-by: James Wen <3b2f4c88287a5fcd7db73a67dd1ab82807db04ee@columbia.edu>

cloudfoundry/go-buildpack,cloudfoundry/go-buildpack,cloudfoundry/go-buildpack