binary_upstart.erb doesn't join args array

Use of `<%= @args %>`results in:
````
 export LOGSTASH_OPTS="["agent", "-f", "/opt/logstash/server/etc/conf.d/", "-l", "/opt/logstash/server/log/logstash.log", "-w", "1"]"
````

so use `@args.join` to get:

````
  export LOGSTASH_OPTS="agent -f /opt/logstash/server/etc/conf.d/ -l /opt/logstash/server/log/logstash.log -w 1"
````