in 5.6 there is no compilation dll - removing pointless references

[twobob]

as pictured. These can be elided. And one kills 5.6

[adrianoc-unity3d]

5.6 is not supported. Recommend version is 2017.3 ~ 2018.1.

Anyway I've wrapped some of those usings with *if UNITY_2017_3_OR_NEWER * so we can show a descriptive message if you open this on Unity 2017.2 or older.