Fix build error and warnings

[philip-peterson]

• Ran cargo fix --bin "pullomatic" to satisfy "warning: trait objects without an explicit dyn are deprecated"
• Specified rustc-serialize version to remove error:

error[E0310]: the parameter type `T` may not live long enough
    --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rustc-serialize-0.3.24/src/serialize.rs:1155:5
     |
1155 |     fn decode<D: Decoder>(d: &mut D) -> Result<Cow<'static, T>, D::Error...
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |     |
     |     the parameter type `T` must be valid for the static lifetime...
     |     ...so that the type `T` will meet its required lifetime bounds...
     |
note: ...that is required by this bound
    --> /rustc/3f5fd8dd41153bc5fdca9427e9e05be2c767ba23/library/alloc/src/borrow.rs:180:30
help: consider adding an explicit lifetime bound

• Fixed unhelpful message: Failed to load config: description() is deprecated; use Display and similar
• Reduced other compile-time warnings

tyia for reviewing!

[philip-peterson]

There are more warnings that won't be resolved until tomaka/rouille#271 is fixed or if pullomatic stops using rouille.