raise default version to 1.16.0

this fixes the acceptance tests. But there is a bigger issue here.
The code in manifests/init.pp is not idempotent:
```
 String[1] $nginx_version                                = pick(fact('nginx_version'), '1.6.0'),
```
Turns out on the first run the fact might not be set yet leading to a pre 1.15.0 compatible
configuration on systems wich ship a newer version of nginx. Leading to
```
nginx: [emerg] unknown directive "ssl" in /etc/nginx/sites-enabled/www.puppetlabs.com.conf:25
```

.rspec

@@ -3,3 +3,4 @@
 
 --format documentation
 --color
+--fail-fast

manifests/init.pp

@@ -240,7 +240,7 @@
   Hash $nginx_upstreams                                   = {},
   Nginx::UpstreamDefaults $nginx_upstreams_defaults       = {},
   Boolean $purge_passenger_repo                           = true,
-  String[1] $nginx_version                                = pick(fact('nginx_version'), '1.6.0'),
+  String[1] $nginx_version                                = pick(fact('nginx_version'), '1.16.0'),
 
   ### END Hiera Lookups ###
 ) inherits nginx::params {