Add nginx 1.15.4, remove nginx 1.15.3
Add nginx 1.15.4, remove nginx 1.15.3

for stack(s) cflinuxfs2, cflinuxfs3 [#160793492]

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