Release version 0.5.0

- HAProxy https://github.com/mackerelio/mackerel-agent-plugins/pull/17
- Varnish https://github.com/mackerelio/mackerel-agent-plugins/pull/18
- Squid https://github.com/mackerelio/mackerel-agent-plugins/pull/19
- Plack (Fixed) https://github.com/mackerelio/mackerel-agent-plugins/pull/20
- SNMP (https://github.com/mackerelio/mackerel-agent-plugins/pull/24)
- EC2 CPUCredis https://github.com/mackerelio/mackerel-agent-plugins/pull/25
- Elasticsearch https://github.com/mackerelio/mackerel-agent-plugins/pull/26  https://github.com/mackerelio/mackerel-agent-plugins/pull/27
- JVM https://github.com/mackerelio/mackerel-agent-plugins/pull/28
- Apache2 (Updated) https://github.com/mackerelio/mackerel-agent-plugins/pull/29
- Linux procs https://github.com/mackerelio/mackerel-agent-plugins/pull/30
- MongoDB https://github.com/mackerelio/mackerel-agent-plugins/pull/31
- ELB https://github.com/mackerelio/mackerel-agent-plugins/pull/32
- RDS https://github.com/mackerelio/mackerel-agent-plugins/pull/35
- PHP-APC https://github.com/mackerelio/mackerel-agent-plugins/pull/37