Updated IE public build script to use MILESTONE_VERSION.
Updated IE public build script to use MILESTONE_VERSION.

Also added comment clarifying that we only use the script for building dependencies at present.

chippey/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,cedriclaunay/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,boberfly/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,hradec/gaffer,andrewkaufman/gaffer,chippey/gaffer,ImageEngine/gaffer,boberfly/gaffer,appleseedhq/gaffer,chippey/gaffer,GafferHQ/gaffer,goddardl/gaffer,GafferHQ/gaffer,chippey/gaffer,ImageEngine/gaffer,goddardl/gaffer,ivanimanishi/gaffer,hradec/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,hradec/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,hradec/gaffer,boberfly/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,cedriclaunay/gaffer,goddardl/gaffer,goddardl/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,hradec/gaffer,cedriclaunay/gaffer