Add nginx 1.15.6, remove nginx 1.15.5
Add nginx 1.15.6, remove nginx 1.15.5

for stack(s) cflinuxfs2, cflinuxfs3 [#161755150]

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