chore(.gitignore) ignore unity project version text file
chore(.gitignore) ignore unity project version text file

The `ProjectSettings/ProjectVersion.txt` has been removed from the
repo and should not be checked in as different versions of Unity can
conflict with this file so it should only be available locally.

thacio/SteamVR_Unity_Toolkit,red-horizon/VRTK,phr00t/SteamVR_Unity_Toolkit,gomez-addams/SteamVR_Unity_Toolkit,thestonefox/SteamVR_Unity_Toolkit,Blueteak/SteamVR_Unity_Toolkit,jcrombez/SteamVR_Unity_Toolkit,mimminito/SteamVR_Unity_Toolkit,adamjweaver/VRTK,virror/VRTK,gpvigano/SteamVR_Unity_Toolkit,bengolds/SteamVR_Unity_Toolkit,TMaloteaux/SteamVR_Unity_Toolkit,gpvigano/VRTK,thestonefox/VRTK,nmccarty/htvr,mbbmbbmm/SteamVR_Unity_Toolkit,Fulby/VRTK,pargee/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK