blender export vertexgroup holes fixed

Background:
Normaly only the first two (strongest influence on vertex) vertexgroups are exported per vertex. 
If not bone related vertexgroups have stronger influence on a vertex the bone-vertexgroups would be skipped.
This produced holes in the mesh and headaches to users.

Solution:
Appending only bone-vertexgroups to bone_array in exporter on line 858 prevents bone-vertexgroups to be sorted out on line 863.