Update minimum unity editor version supported to 2018.3

Author: fabriziounity

src/package.json

@@ -2,7 +2,7 @@
     "name": "com.unity.mathematics",
     "displayName": "Mathematics",
     "version": "0.0.1-preview.1",
-    "unity": "2018.1",
+    "unity": "2018.3",
     "description": "Unity's C# SIMD math library providing vector types and math functions with a shader like syntax. This package is still in experimental phase.",
     "keywords": [
         "unity"