Fix nagating of which command result in install_server.sh

If you install redis w/ install_server.sh on RHEL(or its flavors), you'll end up with the following error:

```
$ sudo ./install_server.sh
Welcome to the redis service installer
This script will help you easily set up a running redis server
...
Installing service...
./install_server.sh: line 176: update-rc.d: command not found
 exists, process is already running or crashed
Installation successful!
```

This happens because negation of which command result is not handled correctly.