Update unix makefile

libdb4.8 instructions
"version's" -> "versions"

Also I think the reason behind still being on libdb4.8 has to do with binary wallet compatibility (https://bitcointalk.org/index.php?topic=356595.0). If you compile using libdb5+ I think the block database is supposed to upgrade and it's a one-way kind of transformation (should probably see if that's the case in the codez...). 

On the whole, I think we could probably just shorten this section to: "Install libdb4.8 or you're gunna have a bad time."