import crash when normals not present

[gamemachine]

This can be fixed in triangle_from_face_components by checking for the existence of a normals index in the same way you do a textures index.

normal = normal_index ? normals[normal_index-1] : nil

[MishaConway]

Thanks for noting the issue. I am on vacation right now and don't have access to a development computer. You can either fix this in a pull request or when I get back I will fix it and push a new version of the rubygem.

[MishaConway]

@gamemachine Do you have leeway to implement this change in a pull request?