Reflect api changes in Alembic_1.1.2+

This changes to IECoreAlembic allow it to build against Alembic 1.1.2 and newer.

From the Alembic 1.1.2 release notes...
> Child bounds are no longer written and read via the schema's sample class, 
> instead the child bounds property is directly exposed on the schema. 
> Normals and UVs IGeomParams are no longer returned by reference. 

Also includes a minor fix to the SConstruct. It adds a missing dependency to the Python module ensuring correct behaviour for multicore builds.