Out-of-date wiki build instructions for OSX 10.8

[elavoie]

I tried building the 'next' branch of the MaidSafe super-project on OSX 10.8.5 with Clang 4.2. However, the associated libc++ library has private constructors for lock_guards which prevents building the Passport library. I updated to Clang 5.1 by downloading the Command Line Tools rev. April 2014 from Apple and the build is now fine.

The wiki entry should be updated to:

• Clang minimum version 5.1 (install the Command Line Tools either from XCode or by downloading them directly from Apple Developer's site, make sure to have revision April 2014 or more recent)