Add nginx 1.15.12, remove nginx 1.15.11
Add nginx 1.15.12, remove nginx 1.15.11

for stack(s) cflinuxfs2, cflinuxfs3 [#165387664]

cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack