[Bf-cycles] Switch to OSL 1.5

[Thomas Dinges]

Hi,
OSL 1.5 is about to be released. You can checkout the "RB-1.5" branch or 
download the "Release-1.5.9RC2" archive: 
https://github.com/imageworks/OpenShadingLanguage

* Their API changed a bit, probably needs a few fixes on our end. I can 
take a look at that later this week.
* As far as I know (Sergey, please correct me if wrong), we would like 
to jump to LLVM 3.4, and be consistent here on all platforms (we use 
different llvm versions on Windows, Mac.. now).

I would appreciate it, if release builders could take a look at this, 
this way compiler errors can still be reported upstream before they tag 
the final release.

Best regards,
Thomas