Patch for [SeeRM #8315]

My try to solve http://dev.metasploit.com/redmine/issues/8315, see the RM ticket for explanations.

* Test after the patch, on a macosx system, where there isn't binay for by default, and nothing is configured manually:

```
msf auxiliary(jtr_linux) > run

[*] Seeding wordlist with DB schema info... 0 words added
[*] Seeding with MSSQL Instance Names....0 words added
[*] Seeding with hostnames....0 words added
[*] Seeding with found credentials....22 words added
[*] Seeding with cracked passwords from John....0 words added
[*] Seeding with default John wordlist...88395 words added
[*] De-duping the wordlist....
[*] Wordlist Seeded with 88407 words
[*] HashList: /var/folders/36/syfyhgjs5g7679944nxfn3lw0000gn/T/jtrtmp20130819-4727-1dh6r7d
[*] Trying Format:md5 Wordlist: /var/folders/36/syfyhgjs5g7679944nxfn3lw0000gn/T/jtrtmp20130819-4727-f9mmu1
[-] John the Ripper executable not found
[*] Trying Format:md5 Rule: All4...
[-] John the Ripper executable not found
[*] Trying Format:md5 Rule: Digits5...
[-] John the Ripper executable not found
[*] Trying Format:des Wordlist: /var/folders/36/syfyhgjs5g7679944nxfn3lw0000gn/T/jtrtmp20130819-4727-f9mmu1
[-] John the Ripper executable not found
[*] Trying Format:des Rule: All4...
[-] John the Ripper executable not found
[*] Trying Format:des Rule: Digits5...
[-] John the Ripper executable not found
[*] Trying Format:bsdi Wordlist: /var/folders/36/syfyhgjs5g7679944nxfn3lw0000gn/T/jtrtmp20130819-4727-f9mmu1
[-] John the Ripper executable not found
[*] Trying Format:bsdi Rule: All4...
[-] John the Ripper executable not found
[*] Trying Format:bsdi Rule: Digits5...
[-] John the Ripper executable not found
[-] John the Ripper executable not found
[*] 0 hashes were cracked!
[*] Auxiliary module execution completed

```