Discontinue support for Unity 2018.3 and below

[Fydar]

Launching a new version of my asset using Unity's new publishers portal will mean that I will be discontinuing support for any version below 2017.4.

This will mean official discounting support for:

• Unity 2018.2.0
• Unity 2017.3.0
• Unity 5.4.0
• Unity 5.3.0

All pre-existing compatibility shims for these old versions will continue to exist in the codebase and the asset should remain compatible with these versions. However, the asset will no longer be built against these versions of Unity.