Replace problematic libyaml

This aims replace the libyaml located at external/darwin64.

The problem is that `libyaml.a` (which is linked to `nupic.core`) was build to `libstdc++` and not `libc++` (which is the c++ standard library at OSX). Because this we are seeing several errors related to OSX at nupic.core.

To I find this, I typed:

`nm /Users/davidragazzi/Desktop/DavidRagazzi/nupic/extensions/core/build/release/lib/libyaml.a | grep 'U '`

and realised that `libyaml.a` that was the only library at nupic.core (I tested all at nupic.core/libs) that requires `__ZNSt8ios_base4InitD1Ev` symbol which is provided by `libstdc++.a`, and not `libc++.a`.