Fixing deprecated compiler constructs

[maciej-makowski]

Seems like dyn has to be explicitly stated in the current version of the compiler.

   Compiling giphy v0.3.0 (C:\Users\macie\Projects\giphy-rs)
warning: trait objects without an explicit `dyn` are deprecated
  --> src\v1\async.rs:36:14
   |
36 |     ) -> Box<Future<Item = ResponseType, Error = reqwest::Error> + Send>;
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Future<Item = ResponseType, Error = reqwest::Error> + Send`
   |
   = note: `#[warn(bare_trait_objects)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2021 edition!
   = note: for more information, see issue #80165 <https://github.com/rust-lang/rust/issues/80165>

warning: trait objects without an explicit `dyn` are deprecated
  --> src\v1\async.rs:47:14
   |
47 |     ) -> Box<Future<Item = ResponseType, Error = reqwest::Error> + Send> {
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Future<Item = ResponseType, Error = reqwest::Error> + Send`
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2021 edition!
   = note: for more information, see issue #80165 <https://github.com/rust-lang/rust/issues/80165>

warning: 2 warnings emitted