Update Unity.gitignore (#2502)

Prevents directories: `Library`
`Temp`
`Obj`
`Build`
`Builds`
`Assets/AssetStoreTools`
from being included in the remote if the project sits in the `Assets` folder.

Unity.gitignore

-/[Ll]ibrary/
-/[Tt]emp/
-/[Oo]bj/
-/[Bb]uild/
-/[Bb]uilds/
-/Assets/AssetStoreTools*
+*/[Ll]ibrary/
+*/[Tt]emp/
+*/[Oo]bj/
+*/[Bb]uild/
+*/[Bb]uilds/
+*/Assets/AssetStoreTools*
 
 # Visual Studio 2015 cache directory
 /.vs/