Fix duplicate macro import

Build fails on nightly:

   Compiling geocoding v0.0.4 (https://github.com/georust/geocoding#22c593a0)
error[E0252]: the name `Deserialize` is defined multiple times
  --> /home/uk/.cargo/git/checkouts/geocoding-45c790e74d3855a2/22c593a/src/lib.rs:27:5
   |
26 | use serde::{Deserialize, Deserializer};
   |             ----------- previous import of the macro `Deserialize` here
27 | use serde_derive::Deserialize;
   | ----^^^^^^^^^^^^^^^^^^^^^^^^^-
   | |   |
   | |   `Deserialize` reimported here
   | help: remove unnecessary import
   |
   = note: `Deserialize` must be defined only once in the macro namespace of this module

error: aborting due to previous error

For more information about this error, try `rustc --explain E0252`.
error: Could not compile `geocoding`.

georust / geocoding

Fix duplicate macro import #18