Update ui tests to 2018 edition

test_suite/tests/ui/with-variant/skip_ser_struct_field.stderr

@@ -1,12 +1,12 @@
 error: variant `Struct` cannot have both #[serde(serialize_with)] and a field `f1` marked with #[serde(skip_serializing)]
-  --> $DIR/skip_ser_struct_field.rs:6:5
+  --> $DIR/skip_ser_struct_field.rs:5:5
    |
-6  | /     #[serde(serialize_with = "serialize_some_other_variant")]
-7  | |     Struct {
-8  | |         #[serde(skip_serializing)]
-9  | |         f1: String,
-10 | |         f2: u8,
-11 | |     },
+5  | /     #[serde(serialize_with = "serialize_some_other_variant")]
+6  | |     Struct {
+7  | |         #[serde(skip_serializing)]
+8  | |         f1: String,
+9  | |         f2: u8,
+10 | |     },
    | |_____^
 
 error: aborting due to previous error