Use lightweigth bourne shell when possible

This script does not use bash specific features, therfore bash is not
necessary. If this script gets called too often, it may use up
unneccessary resources on sensor host.

For reference, here is comparison (binary size) on MacOS (almost ten
times difference):

```
% ls -lh /bin/sh /bin/bash
-r-xr-xr-x  1 root  wheel   1.3M Feb  9 10:39 /bin/bash
-rwxr-xr-x  1 root  wheel   131K Feb  9 10:39 /bin/sh
```

and on FreeBSD (~5x):

```
% ls -lh /bin/sh /usr/local/bin/bash
-r-xr-xr-x  1 root  wheel   166K Feb 23 20:52 /bin/sh
-rwxr-xr-x  1 root  wheel   963K Dec 15 02:15 /usr/local/bin/bash
```

src/notify_about_attack.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 
 #
 # Hello, lovely FastNetMon customer. I'm really happy to see you here