Fix Nightly Build

[KodrAus]

Depends on: serde-deprecated/aster#72

[KodrAus]

aster should be right now. Update versions and try again

[KodrAus]

See the following warnings:

<elastic_types macros>:19:16: 19:35 warning: private type in public interface, #[warn(private_in_public)] on by default
<elastic_types macros>:19 type Visitor = FieldMappingVisitor ; fn data_type (  ) -> & 'static str {
                                         ^~~~~~~~~~~~~~~~~~~
<elastic_types macros>:56:1: 58:54 note: in this expansion of impl_object_mapping! (defined in <elastic_types macros>)
benches/mod.rs:186:2: 186:106 note: in this expansion of impl_object_mapping! (defined in <elastic_types macros>)
<elastic_types macros>:19:16: 19:35 warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!