Fix warning about implicit dyn being deprecated

[brainplot]

Rust made trait objects with implicit dyn keyword deprecated. This caused a warning during compilation:

warning: trait objects without an explicit `dyn` are deprecated
  --> src/types.rs:60:45
   |
60 |     pub fn all_sources<'a>(&'a self) -> Box<Iterator<Item=&Source> + 'a> {
   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `
dyn Iterator<Item=&Source> + 'a`
   |
   = note: `#[warn(bare_trait_objects)]` on by default

this patch fixes that by adding the dyn keyword

[dan-t]

Thanks!