Change OSX build to download specific SDK from server

Created by: wilsonmr

closes #488 (closed)

The CI has been warning for sometime that the xcode6.4 image was deprecated as of January this year, I don't know exactly what it was doing instead but I think finally it has silently changed to a different version of xcode

As per #323 (closed) specifically: https://github.com/NNPDF/nnpdf/issues/323#issuecomment-434385399 xcode 6.4 comes with the correct SDK for the conda compilers and so if we were using a different xcode then it would explain why the build was failing because the compilers are useless without it

A while ago I said I'd try to make a conda package of the SDK but firstly I couldn't manage and my issue I opened regarding this remains ignored.. Also I think that packaging this is super overkill since in theory one just needs to download and unpack the SDK. As a result I uploaded the SDK to the server - location can of course change it was just a test**

Ok so then this PR gets the osx build to download this SDK and then unpack it into the root of the conda directory and then makes sure that the CONDA_BUILD_SYSROOT is passed down to conda-build

**in fact it would be nice if it worked with vp-get but that's a seperate issue which I could address later - of course adding to the installation documentation ;)