fix nginx install on centos (#336)

fix nginx install on centos (#336)

.pkgr.yml

@@ -2,11 +2,9 @@ notifications: false
 targets:
   centos-6:
     dependencies:
-      - nginx
       - postgresql-server
   centos-7:
     dependencies:
-      - nginx
       - postgresql-server
   debian-8:
     dependencies:

contrib/packager.io/preinstall.sh

@@ -5,8 +5,10 @@
 
 PATH=/opt/zammad/bin:/opt/zammad/vendor/bundle/bin:/sbin:/bin:/usr/sbin:/usr/bin:
 
-# install epel-release on centos (needed for nginx)
+# install epel-release & nginx on centos because it does not work via dependencies
 if [ -n "$(which yum)" ]; then
     yum install -y epel-release
+
+    yum install -y nginx
 fi