Ignore unneeded *.blend1 blender files

**Reasons for making this change:**
By default, the Unity game engine supports the importing of Blender *.blend files and converts them to FBX internally (see documentation link below). Sometimes, blender stores *.blend1 files as a backup copy of your current blender file *.blend. Due to the nature of version control, redundant *.blend1 files do not need to be committed.

**Links to documentation supporting these rule changes:**
[Unity supported file formats](https://docs.unity3d.com/Manual/3D-formats.html)