TerrainFactory throws exception with non-flat height

[david-rhodes]

No terrain is generated:

ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
System.Collections.Generic.List`1[UnityEngine.Vector3].get_Item (Int32 index) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:633)
Mapbox.Unity.MeshGeneration.Factories.TerrainFactory.GenerateTerrainMesh (Mapbox.Unity.MeshGeneration.Data.UnityTile tile, Single heightMultiplier) (at Assets/Mapbox/Core/Unity/MeshGeneration/Factories/TerrainFactory.cs:184)
Mapbox.Unity.MeshGeneration.Factories.TerrainFactory+<CreateTerrainHeight>c__AnonStorey9.<>m__F () (at Assets/Mapbox/Core/Unity/MeshGeneration/Factories/TerrainFactory.cs:125)
Mapbox.Map.Tile.HandleTileResponse (Response response) (at Assets/Mapbox/Core/mapbox-sdk-cs/Map/Tile.cs:174)
Mapbox.Unity.Utilities.HTTPRequest+<DoRequest>c__Iterator2.MoveNext () (at Assets/Mapbox/Core/Unity/Utilities/HTTPRequest.cs:56)
Mapbox.Unity.Utilities.Runnable+Routine.MoveNext () (at Assets/Mapbox/Core/Unity/Utilities/Runnable.cs:130)
UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at /Users/builduser/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)

[david-rhodes]

Fixed: #38