Reduce json <=> jsonb casts from explicit-only to assignment level.

There's no reason to make users write an explicit cast to store a
json value in a jsonb column or vice versa.

We could probably even make these implicit, but that might open us up
to problems with ambiguous function calls, so for now just do this.

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201503011
+#define CATALOG_VERSION_NO	201503031
 
 #endif

src/include/catalog/pg_cast.h

@@ -360,7 +360,7 @@ DATA(insert ( 1562 1562 1687 i f ));
 DATA(insert ( 1700 1700 1703 i f ));
 
 /* json to/from jsonb */
-DATA(insert ( 114 3802 0 e i ));
-DATA(insert ( 3802 114 0 e i ));
+DATA(insert (  114 3802    0 a i ));
+DATA(insert ( 3802  114    0 a i ));
 
 #endif   /* PG_CAST_H */