Remove need to specify lib path

Similar to your other plugin check_nginx you could use the FindBin to avoid the need to change the path.

This:

```
# Nagios specific
# Update Nagios Plugin path according to your platform/installation
use lib "/usr/local/nagios/libexec";
```

Could become:

```
# Nagios specific
use FindBin;
use lib $FindBin::Bin;
```
