Ignore derivable_impls clippy false positive

rust-lang/rust-clippy#7655

    error: this `impl` can be derived
       --> src/map.rs:254:1
        |
    254 | / impl Default for Map<String, Value> {
    255 | |     #[inline]
    256 | |     fn default() -> Self {
    257 | |         Map {
    ...   |
    260 | |     }
    261 | | }
        | |_^
        |
        = note: `#[deny(clippy::derivable_impls)]` implied by `#[deny(clippy::all)]`
        = help: try annotating `map::Map` with `#[derive(Default)]`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#derivable_impls

src/map.rs

@@ -251,6 +251,7 @@ impl Map<String, Value> {
     }
 }
 
+#[allow(clippy::derivable_impls)] // clippy bug: https://github.com/rust-lang/rust-clippy/issues/7655
 impl Default for Map<String, Value> {
     #[inline]
     fn default() -> Self {