Upgrade gin-gonic to latest master, fix compatibility issues

vendor/github.com/gin-gonic/gin/binding/example/test.proto

@@ -0,0 +1,12 @@
+package example;
+
+enum FOO {X=17;};
+
+message Test {
+   required string label = 1;
+   optional int32 type = 2[default=77];
+   repeated int64 reps = 3;
+   optional group OptionalGroup = 4{
+     required string RequiredField = 5;
+   }
+}