Make inverseBindMatrices optional

[danwillm]

The spec mentions that inverseBindMatrices in skins are optional, but tinygltf produces an error when trying to load a model with none defined.

Each skin is defined by a REQUIRED joints property that lists the indices of nodes used as
joints to pose the skin and an OPTIONAL inverseBindMatrices property that points to
an accessor with inverse bind matrices data used to bring coordinates being skinned into 
the same space as each joint.