fix: compile error in 2018.2

Scripts/UIParticleUpdater.cs

@@ -211,9 +211,11 @@ namespace Coffee.UIExtensions
                     Profiler.EndSample();
                 }
 
+#if UNITY_2018_3_OR_NEWER
                 // #102: Do not bake particle system to mesh when the alpha is zero.
                 if (Mathf.Approximately(particle.canvasRenderer.GetInheritedAlpha(), 0))
                     continue;
+#endif
 
                 // Bake main particles.
                 var r = currentPs.GetComponent<ParticleSystemRenderer>();