Bump minimum editor version to 2018.4 and add validation exception.

Mathematics 1.2.1 was released with 2018.3 support but we no longer
support it. Simply bumping the minimum editor version for 1.2.2 will
cause a validation error, but an exception can be added so we can
still release a patch version for the currently supported editors.

Author: unpacklo

src/ValidationExceptions.json

@@ -0,0 +1,9 @@
+{
+    "ErrorExceptions": [
+        {
+            "ValidationTest": "Package Unity Version Validation",
+             "PackageVersion": "1.2.2"
+        }
+    ],
+    "WarningExceptions": []
+}

src/ValidationExceptions.json.meta

@@ -2,7 +2,7 @@
     "name": "com.unity.mathematics",
     "displayName": "Mathematics",
     "version": "1.2.2",
-    "unity": "2018.3",
+    "unity": "2018.4",
     "description": "Unity's C# SIMD math library providing vector types and math functions with a shader like syntax.",
     "keywords": [
         "unity"