Use embedded Ruby, instead of system-wide Ruby.

```
 default["sensu"]["log_directory"]         = "/var/log/sensu"
 default["sensu"]["log_level"]             = "info"
 default["sensu"]["use_ssl"]               = true
-default["sensu"]["use_embedded_ruby"]     = false
+default["sensu"]["use_embedded_ruby"]     = true
 default["sensu"]["init_style"]            = "sysv"
 default["sensu"]["service_max_wait"]      = 10
```

This commit refs #47.
Install gems, using /opt/sensu/embedded/bin/gem.

This commit refs #47.
Use the sensu user's PATH, when using `sudo` command.

This commit refs #47
Delete unnecessary `sudo` command.

This commit refs #47.
Making sure that installing gems via /opt/sensu/embedded/bin/gem after installing sensu.

This commit refs #48.
Merge pull request #48 from kazu634/id/48

[sensu-custom] Gemインストールの順番を変更する